Enable authenticated media by default (#17889)

changelog.d/17889.feature

docs/upgrade.md

docs/usage/configuration/config_documentation.md

synapse/config/repository.py

tests/media/test_media_storage.py

tests/replication/test_multi_media_repo.py

tests/rest/admin/test_admin.py

tests/rest/admin/test_media.py

tests/rest/admin/test_user.py

tests/rest/media/test_domain_blocking.py

tests/rest/media/test_url_preview.py