Use more correct changelog entries for refactoring Generator usage

[MadLittleMods]

Use more correct changelog entries for refactoring Generator usage

• Remove Generator in _quarantine_media_txn() #17813
• Remove Generator in _prune_old_outbound_device_pokes #17814
• Remove Generator in _purge_unreferenced_state_groups twice #17815
• Remove Generator in update_cached_last_access_time #17816
• Remove Generator in store_search_entries_txn #17817

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 (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[MadLittleMods]

Updated all of these because the PR's for these don't actually help with data loss at all.

The PR's simply update a few spots so down the line we can update the type hints to be explicit which will help with us making mistakes in the future.

Context: matrix-org/synapse#15439

To be clear, they didn't fix any bugs or were a problem before. But changing them to be a Sequence will allow us to update the type hint to be Sequence instead of Any

[MadLittleMods]

@devonh (maintainer ping) We should merge this one before a release tomorrow so we don't have to deal with weird changelog conflicts.