fix(Calendar): re-remove aria-readonly on grid cells

[smhigley]

Please verify that:

• Code is up-to-date with the master branch
• Your changes are covered by tests (if possible)
• You've run yarn change locally

I unintentionally un-did the CalendarDayGridCell aria-readonly fix from this PR: #20324 in a simultaneous calendar fix PR. This re-fixes the original fix.

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Sandbox	Source
@fluentui/react 8 starter	Configuration
@fluentui/react-components 9 starter	Configuration

[fabricteam]

📊 Bundle size report

_{🤖 This report was generated against 29edad19a40ed89ef422027bb8ee42b842a09e4b}

[size-auditor]

Asset size changes

Project	Bundle	Baseline Size	New Size	Difference
office-ui-fabric-react	fluentui-react-Calendar	115.287 kB	115.264 kB	-23 bytes
office-ui-fabric-react	fluentui-react-DatePicker	173.503 kB	173.48 kB	-23 bytes
office-ui-fabric-react	fluentui-react-WeeklyDayPicker	96.583 kB	96.56 kB	-23 bytes

 Over Tolerance (1024 B)  Over Baseline  Below Baseline  New   Removed 1 kB = 1000 B

Baseline commit: 29edad19a40ed89ef422027bb8ee42b842a09e4b (build)

[fabricteam]

Perf Analysis (@fluentui/react)

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
ContextualMenu	mount	14286	7108	1000	Possible regression

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
Avatar	mount	773	769	5000
BaseButton	mount	809	805	5000
Breadcrumb	mount	2035	2351	1000
ButtonNext	mount	437	455	5000
Checkbox	mount	1324	1304	5000
CheckboxBase	mount	1142	1134	5000
ChoiceGroup	mount	4145	4059	5000
ComboBox	mount	874	841	1000
CommandBar	mount	8608	8650	1000
ContextualMenu	mount	14286	7108	1000	Possible regression
DefaultButton	mount	988	998	5000
DetailsRow	mount	3221	3172	5000
DetailsRowFast	mount	3295	3249	5000
DetailsRowNoStyles	mount	3019	3024	5000
Dialog	mount	2252	2216	1000
DocumentCardTitle	mount	174	171	1000
Dropdown	mount	2728	2771	5000
FluentProviderNext	mount	1609	1522	5000
FluentProviderWithTheme	mount	133	131	10
FluentProviderWithTheme	virtual-rerender	87	112	10
FluentProviderWithTheme	virtual-rerender-with-unmount	176	187	10
FocusTrapZone	mount	1613	1592	5000
FocusZone	mount	1603	1589	5000
IconButton	mount	1433	1507	5000
Label	mount	329	276	5000
Layer	mount	2525	2537	5000
Link	mount	386	419	5000
MakeStyles	mount	1432	1479	50000
MenuButton	mount	1279	1268	5000
MessageBar	mount	1737	1737	5000
Nav	mount	2731	2782	1000
OverflowSet	mount	988	914	5000
Panel	mount	2186	2165	1000
Persona	mount	745	751	1000
Pivot	mount	1247	1237	1000
PrimaryButton	mount	1094	1113	5000
Rating	mount	6492	6253	5000
SearchBox	mount	1182	1149	5000
Shimmer	mount	2158	2168	5000
Slider	mount	1706	1678	5000
SpinButton	mount	4257	4259	5000
Spinner	mount	385	393	5000
SplitButton	mount	2643	2696	5000
Stack	mount	460	488	5000
StackWithIntrinsicChildren	mount	1924	1972	5000
StackWithTextChildren	mount	4331	4518	5000
SwatchColorPicker	mount	9715	9838	5000
TagPicker	mount	2336	2306	5000
TeachingBubble	mount	11277	11148	5000
Text	mount	343	394	5000
TextField	mount	1205	1217	5000
ThemeProvider	mount	1051	1043	5000
ThemeProvider	virtual-rerender	553	550	5000
ThemeProvider	virtual-rerender-with-unmount	1599	1611	5000
Toggle	mount	714	715	5000
buttonNative	mount	124	132	5000

Perf Analysis (@fluentui/react-northstar)

Perf tests with no regressions

Scenario	Current PR Ticks	Baseline Ticks	Ratio
ButtonMinimalPerf.default	138	121	1.14:1
ChatDuplicateMessagesPerf.default	257	225	1.14:1
HeaderSlotsPerf.default	657	599	1.1:1
LoaderMinimalPerf.default	581	528	1.1:1
ChatMinimalPerf.default	624	575	1.09:1
AttachmentSlotsPerf.default	899	833	1.08:1
AvatarMinimalPerf.default	178	167	1.07:1
ListWith60ListItems.default	572	536	1.07:1
RefMinimalPerf.default	210	197	1.07:1
BoxMinimalPerf.default	301	284	1.06:1
ListCommonPerf.default	546	516	1.06:1
DatepickerMinimalPerf.default	4696	4459	1.05:1
ImageMinimalPerf.default	319	304	1.05:1
ButtonSlotsPerf.default	480	463	1.04:1
FormMinimalPerf.default	359	345	1.04:1
HeaderMinimalPerf.default	320	308	1.04:1
ItemLayoutMinimalPerf.default	1033	992	1.04:1
ListMinimalPerf.default	442	424	1.04:1
ListNestedPerf.default	479	460	1.04:1
MenuMinimalPerf.default	710	685	1.04:1
SkeletonMinimalPerf.default	298	287	1.04:1
SliderMinimalPerf.default	1441	1380	1.04:1
AlertMinimalPerf.default	244	237	1.03:1
ChatWithPopoverPerf.default	285	276	1.03:1
CheckboxMinimalPerf.default	2263	2199	1.03:1
TableMinimalPerf.default	348	338	1.03:1
TreeMinimalPerf.default	690	671	1.03:1
TreeWith60ListItems.default	159	154	1.03:1
VideoMinimalPerf.default	538	524	1.03:1
AnimationMinimalPerf.default	463	452	1.02:1
CardMinimalPerf.default	479	468	1.02:1
DropdownMinimalPerf.default	2576	2536	1.02:1
EmbedMinimalPerf.default	3494	3432	1.02:1
MenuButtonMinimalPerf.default	1418	1392	1.02:1
SplitButtonMinimalPerf.default	3630	3569	1.02:1
TextMinimalPerf.default	303	297	1.02:1
CarouselMinimalPerf.default	404	399	1.01:1
DialogMinimalPerf.default	651	644	1.01:1
DividerMinimalPerf.default	308	306	1.01:1
InputMinimalPerf.default	1086	1075	1.01:1
LabelMinimalPerf.default	331	328	1.01:1
PopupMinimalPerf.default	526	520	1.01:1
PortalMinimalPerf.default	155	153	1.01:1
TextAreaMinimalPerf.default	428	422	1.01:1
ToolbarMinimalPerf.default	800	789	1.01:1
TooltipMinimalPerf.default	874	863	1.01:1
FlexMinimalPerf.default	251	251	1:1
SegmentMinimalPerf.default	298	297	1:1
TableManyItemsPerf.default	1592	1599	1:1
ButtonOverridesMissPerf.default	1371	1386	0.99:1
LayoutMinimalPerf.default	307	309	0.99:1
StatusMinimalPerf.default	561	567	0.99:1
DropdownManyItemsPerf.default	564	573	0.98:1
ProviderMergeThemesPerf.default	1438	1471	0.98:1
CustomToolbarPrototype.default	3368	3431	0.98:1
ProviderMinimalPerf.default	931	962	0.97:1
RadioGroupMinimalPerf.default	370	380	0.97:1
GridMinimalPerf.default	271	284	0.95:1
RosterPerf.default	953	1002	0.95:1
AccordionMinimalPerf.default	129	138	0.93:1
AttachmentMinimalPerf.default	124	133	0.93:1
ReactionMinimalPerf.default	288	321	0.9:1
IconMinimalPerf.default	468	521	0.9:1