Use receipts `event_stream_ordering` column, remove joins to `events`

This issue has been migrated from [#13918](https://github.com/matrix-org/synapse/pull/13918).

---

Follow up to: https://github.com/matrix-org/synapse/commit/cdbb6412327b542e0dead792717fe58253291131

This relies on the completion of the background task introduced by the above change. I suggest that we leave this in draft for N(?) releases and then force the migration similar to https://github.com/matrix-org/synapse/commit/da41a7cd618d11b05c2c04c39068fd4b1e1b7894#diff-ddb3c5844daede09acca491cc04a3740730179fca1fa75aa25b921727209947a, probably with an appropriate upgrade note at the time.

Signed off by Nick @ Beeper (@fizzadar).

### Pull Request Checklist

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog file](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#changelog).
* [x] Pull request includes a [sign off](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#sign-off)
* [x] [Code style](https://matrix-org.github.io/synapse/latest/code_style.html) is correct
(run the [linters](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

Start by reading the referenced background-task commit and the follow-up migration commit, then trace the receipts queries that still join events. Confirm the background task has completed before changing the rollout. Done means receipts use event_stream_ordering without those joins, with an appropriate upgrade note when the migration is forced.

Written by the indexing model from the issue text.