Set thread_id column to non-null for event_push_{actions,actions_staging,summary}

[clokep]

Updates the schema to require a thread_id (i.e. add a constraint that the column is non-null) for event_push_actions, event_push_actions_staging, and event_push_actions_summary.

This bumps the schema compat version to 74. This was last bumped in #15036, which was released in v1.77.0rc2 so users will be unable to rollback to earlier than that version.

This has a few commits, it performs the following operations:

• Re-applies Set thread_id column to non-null for event_push_{actions,actions_staging,summary} #15350 (which itself reapplies part of Track notification counts per thread (implement MSC3773) (redo) #13776).
• Re-applies Drop IS NULL indices *after* the ALTER TABLE statement. #15357.
• Makes the changes suggested in Re-apply making thread_id required in push rules tables #15359 to switch from altering the column to using a NOT VALID constraint.

Fixes #14225
Fixes #15359

[clokep]

I'm not really sure how to test that this won't lock the table / cause matrix.org issues, but the documentation sounds like it should work?

[clokep]

Shout-out to @reivilibre for figuring out this approach, by the way!

[clokep]

@erikjohnston I think this is all set now, pending CI being happy.