Squash migrations to 20211202041233

What does this MR do and why?

See !105695 (merged) and !108717 (merged) for context. We are squashing migrations up to 20211202041233, this MR is identical in purpose to those.

There are two specific steps that I had to take that are unique to this squash that I want to draw attention to, given that the changes introduced are extensive:

"Deprecated" migration

There is a spec test that is dependent upon a migration that was squashed. We do not need the migration to remain in its normal spot, but it must be referred to for the test to pass. Therefore, I moved it into a 'deprecated_migrations' folder in /db. Is there is better place for this to live?

"Nerfed" tests

There are some tests that were introduced during an earlier schema version, and the schema we're squashing to is no longer compatible with the tests. The tests are for background/batched background migrations that were started before the squash date. Here is an example.

Overall, I have tried to preserve tests where it makes sense - every spot where a specific schema is required and that schema is before our squash date, I update it and ensure the test passes, the above two examples are the outliers where I had to have a different solution.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #370640 (closed)