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/dialogs/__snapshots__/MessageEditHistoryDialog-test.tsx.snap

@@ -48,7 +48,6 @@ exports[`<MessageEditHistory /> should match the snapshot 1`] = `
                 aria-label="Thu, Jan 1 1970"
                 class="mx_DateSeparator"
                 role="separator"
-                tabindex="-1"
               >
                 <hr
                   role="none"
@@ -165,7 +164,6 @@ exports[`<MessageEditHistory /> should support events with 1`] = `
                 aria-label=",  NaN NaN"
                 class="mx_DateSeparator"
                 role="separator"
-                tabindex="-1"
               >
                 <hr
                   role="none"