[Maps] Remove use of deprecated $opacity

[thomasneirynck]

$opacity is deprecated in the upcoming Amsterdam EUI theme.

This is now just producing spam at build-time.

13:44:27     │ warn worker stderr WARNING: The $opacity variable of euiBottomShadowMedium() will be depricated in a future version of EUI.
13:44:27     │ warn worker stderr          on line 35 of node_modules/@elastic/eui/src/themes/eui-amsterdam/global_styling/mixins/_shadow.scss, in mixin `euiBottomShadowMedium`
13:44:27     │ warn worker stderr          from line 12 of src/plugins/maps_legacy/public/map/_leaflet_overrides.scss
13:44:27     │ warn worker stderr          from line 3 of stdin
13:44:27     │ warn worker stderr 
13:44:27     │ warn worker stderr WARNING: The $opacity variable of euiBottomShadowMedium() will be depricated in a future version of EUI.
13:44:27     │ warn worker stderr          on line 35 of node_modules/@elastic/eui/src/themes/eui-amsterdam/global_styling/mixins/_shadow.scss, in mixin `euiBottomShadowMedium`
13:44:27     │ warn worker stderr          from line 3 of src/plugins/maps_legacy/public/map/_legend.scss
13:44:27     │ warn worker stderr          from line 4 of stdin
13:44:27     │ warn worker stderr 

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: aa9647a
• Storybooks Preview

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
triggersActionsUi	1.6MB	1.5MB	-23.9KB

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
triggersActionsUi	104.0KB	104.1KB	+82.0B

Unknown metric groups

async chunk count

id	before	after	diff
triggersActionsUi	41	42	+1

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[elizabetdev]

LGTM! 🎉

By default, the mixin euiBottomShadowMedium() returns an opacity of .2 which doesn't make much difference.