Convert status codes to `HTTPStatus` in `synapse.rest.admin` (#11452)

changelog.d/11452.misc

synapse/rest/admin/__init__.py

synapse/rest/admin/_base.py

synapse/rest/admin/devices.py

synapse/rest/admin/event_reports.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/users.py