Use `StrCollection` in more places

This issue has been migrated from [#14809](https://github.com/matrix-org/synapse/issues/14809).

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Using a `Sequence[str]` or `Collection[str]` is bad since a `str` also fits those and you *usually* want this to refer to a `Set[str]`, `List[str]`, `Tuple[str, ...]`, etc.

In #14716 @reivilibre added a `StrCollection` type which we can re-use for this.

This has bit us a few times in the past but I'm failing to find references at the moment.

It would be good to audit `Sequence[str]` and `Collection[str]` and see if any of those should be updated.

Start by searching the Synapse Python codebase for annotations using Sequence[str] and Collection[str], then review each occurrence in the context of its callers. Compare candidates with the StrCollection type added in #14716 and update only cases where accepting a plain str is undesirable. Done means the audit is complete and the appropriate annotations are consistently changed.

Written by the indexing model from the issue text.