Use StrCollection in additional places. (#16301)

changelog.d/16301.misc

synapse/app/_base.py

synapse/config/_base.py

synapse/events/__init__.py

synapse/events/builder.py

synapse/events/validator.py

synapse/http/client.py

synapse/http/servlet.py

synapse/metrics/__init__.py

synapse/notifier.py

synapse/rest/client/_base.py

synapse/state/__init__.py

synapse/state/v1.py

synapse/state/v2.py

synapse/storage/databases/main/event_federation.py

synapse/visibility.py