Convert status codes to `HTTPStatus` in `tests.rest.admin` (#11455)

changelog.d/11455.misc

tests/rest/admin/test_admin.py

tests/rest/admin/test_background_updates.py

tests/rest/admin/test_device.py

tests/rest/admin/test_event_reports.py

tests/rest/admin/test_media.py

tests/rest/admin/test_registration_tokens.py

tests/rest/admin/test_room.py

tests/rest/admin/test_server_notice.py

tests/rest/admin/test_statistics.py

tests/rest/admin/test_user.py

tests/rest/admin/test_username_available.py