Clean up `synapse.rest.admin` (#11535)

changelog.d/11535.misc

synapse/rest/admin/__init__.py

synapse/rest/admin/background_updates.py

synapse/rest/admin/devices.py

synapse/rest/admin/event_reports.py

synapse/rest/admin/federation.py

synapse/rest/admin/groups.py

synapse/rest/admin/media.py

synapse/rest/admin/registration_tokens.py

synapse/rest/admin/rooms.py

synapse/rest/admin/server_notice_servlet.py

synapse/rest/admin/statistics.py

synapse/rest/admin/username_available.py

synapse/rest/admin/users.py

tests/rest/admin/test_statistics.py