v7 Calendar cherry-pick: remove explicit readonly semantics from grid and gridcells

[smhigley]

Cherry pick to v7 of #20324, fixes Android Talkback bug.

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 742929c:

Sandbox	Source
Fluent UI Button	Configuration
codesandbox-react-template	Configuration

[size-auditor]

Asset size changes

Project	Bundle	Baseline Size	New Size	Difference
office-ui-fabric-react	office-ui-fabric-react-Calendar	140.461 kB	140.438 kB	-23 bytes
office-ui-fabric-react	office-ui-fabric-react-DatePicker	208.542 kB	208.519 kB	-23 bytes

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

Baseline commit: a45b9f5cf52ad4c559ea3b5c94df9f1752c2aebc (build)

[fabricteam]

Perf Analysis

No significant results to display.

All results

Scenario	Render type	7.0 Ticks	PR Ticks	Iterations	Status
BaseButton	mount	767	748	5000
Breadcrumb	mount	35987	36563	5000
Checkbox	mount	1306	1312	5000
CheckboxBase	mount	1103	1078	5000
ChoiceGroup	mount	4104	4112	5000
ComboBox	mount	770	765	1000
CommandBar	mount	6646	6647	1000
ContextualMenu	mount	12002	12169	1000
DefaultButton	mount	959	951	5000
DetailsRow	mount	3247	3162	5000
DetailsRowFast	mount	3163	3194	5000
DetailsRowNoStyles	mount	2974	3052	5000
Dialog	mount	1914	1927	1000
DocumentCardTitle	mount	1572	1566	1000
Dropdown	mount	2169	2148	5000
FocusTrapZone	mount	1443	1463	5000
FocusZone	mount	1530	1546	5000
IconButton	mount	1465	1482	5000
Label	mount	280	282	5000
Layer	mount	1681	1650	5000
Link	mount	368	369	5000
MenuButton	mount	1218	1229	5000
MessageBar	mount	1739	1747	5000
Nav	mount	2762	2809	1000
OverflowSet	mount	1186	1199	5000
Panel	mount	1252	1263	1000
Persona	mount	806	799	1000
Pivot	mount	1252	1227	1000
PrimaryButton	mount	1091	1080	5000
Rating	mount	6636	6641	5000
SearchBox	mount	1057	1076	5000
Shimmer	mount	2167	2184	5000
Slider	mount	1265	1276	5000
SpinButton	mount	4288	4255	5000
Spinner	mount	344	370	5000
SplitButton	mount	2664	2662	5000
Stack	mount	415	423	5000
StackWithIntrinsicChildren	mount	1444	1444	5000
StackWithTextChildren	mount	4056	4074	5000
SwatchColorPicker	mount	8969	8916	5000
TagPicker	mount	2442	2410	5000
TeachingBubble	mount	43522	43502	5000
Text	mount	348	352	5000
TextField	mount	1177	1155	5000
Toggle	mount	682	696	5000
button	mount	90	99	5000

[smhigley]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).