Fall back to checking power levels when sourcing local restricted join users

[nexy7574]

Fixes 19120 by always falling back to checking power levels for local users if a local creator cannot be found in a v12 room.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
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• Code style is correct (run the linters)

[MadLittleMods]

This kind of change deserves a test of some sort to ensure we don't regress into the future. It's probably easiest to write a Complement test to setup this kind of scenario. Is this something you'd be comfortable with tackling?

Probably something in tests/restricted_rooms_test.go

As an example of running Complement tests for the Synapse project, here is how to run a single test:

COMPLEMENT_DIR=../complement ./scripts-dev/complement.sh ./tests -run TestRestrictedRoomsLocalJoin

(checkout the complement repo as a sibling to the synapse repo)

[nexy7574]

Sure, I'll give it a shot

[nexy7574]

Opened matrix-org/complement#836, let me know if I missed a step or anything.