Remove deprecated `/_matrix/client/*/admin` endpoints (#8785)

UPGRADE.rst

changelog.d/8785.removal

docs/admin_api/user_admin_api.rst

synapse/_scripts/register_new_matrix_user.py

synapse/rest/admin/__init__.py

synapse/rest/admin/_base.py

synapse/rest/admin/groups.py

synapse/rest/admin/media.py

synapse/rest/admin/rooms.py

synapse/rest/admin/users.py

tests/rest/admin/test_admin.py

tests/rest/admin/test_room.py

tests/rest/admin/test_user.py

tests/rest/client/v2_alpha/test_register.py

tests/storage/test_client_ips.py

tests/unittest.py