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/structures/__snapshots__/MessagePanel-test.tsx.snap

@@ -42,7 +42,6 @@ exports[`MessagePanel should handle lots of membership events quickly 1`] = `
             aria-label="Thu, Jan 1 1970"
             class="mx_DateSeparator"
             role="separator"
-            tabindex="-1"
           >
             <hr
               role="none"