Fix v12 rooms when using frozen dicts (#19235)

Fix #19233 

Synapse fails to handle events in v12 rooms when the server is run with
the `{use_frozen_dicts: True}` config.
This PR fixes the issue, and adds tests which cover room creation,
joining, and joining over federation, with both frozen and not frozen
config settings, by extending the existing `test_send_join` federation
tests.

This approach to testing was chosen as it is a simple way to get high
level integration style test coverage, without going through all our
existing tests and trying to retroactively add in coverage when using
frozen dicts.
This should provide an easy place for future room versions to extend the
suite of tests and reduce the chance of introducing subtle bugs like
this in the future.

### Pull Request Checklist

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changelog.d/19235.bugfix

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+Fix v12 rooms when running with `use_frozen_dicts: True`.