Fix multiple accessibility defects identified by AXE (#10606)

* Mark effects overlay canvas as aria hidden

* Ensure date separators aren't seen as focusable aria separators

* Fix

* Fix font slider not having aria label

* Add missing aria labels

* Fix settings flags setting aria-checked={null}

* Update snapshots

test/components/views/messages/__snapshots__/DateSeparator-test.tsx.snap

@@ -6,7 +6,6 @@ exports[`DateSeparator renders the date separator correctly 1`] = `
     aria-label="Today"
     class="mx_DateSeparator"
     role="separator"
-    tabindex="-1"
   >
     <hr
       role="none"
@@ -34,7 +33,6 @@ exports[`DateSeparator when feature_jump_to_date is enabled renders the date sep
     aria-label="Fri, Dec 17 2021"
     class="mx_DateSeparator"
     role="separator"
-    tabindex="-1"
   >
     <hr
       role="none"