feat!: Remove Studio Maintenance & Announcements (openedx#37432)

The announcements editor was never ported to frontend-app-authoring, and
the announcements display was never ported to frontend-app-learner-dashboard.
This announcements feature is rarely used, undocumented, non-a11y-friendly, and
there were no volunteers to port it to the new frontends. It is the last
remaining part of the legacy Studio "Maintenance" dashboard. So, we are
removing it.

BREAKING CHANGE: This removes...
* Studio Maintenance dashboard legacy frontend
* Studio Edit Announcements legacy frontend
* The snippet of legacy learner dashboard which renders announcements
* openedx/features/announcements djangoapp
* The ENABLE_ANNOUNCEMENTS feature flag

Not removed:
* The announcements_announcement table from openedx/features/announcements .
  The table is most likely very small, as it is only populated by administrators. Removing
  it would be more labor for us and more risk of toil for operators than is worthwhile.

Closes: openedx#36263

Author: kdmccormick

.github/workflows/unit-test-shards.json

@@ -239,7 +239,6 @@
       "cms/djangoapps/course_creators/",
       "cms/djangoapps/export_course_metadata/",
       "cms/djangoapps/modulestore_migrator/",
-      "cms/djangoapps/maintenance/",
       "cms/djangoapps/models/",
       "cms/djangoapps/pipeline_js/",
       "cms/djangoapps/xblock_config/",