Bail during start-up if there are old background updates.

This issue has been migrated from [#16397](https://github.com/matrix-org/synapse/pull/16397).

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This is a basic implementation to enforce that some old background updates have run.

My hope is that this could be used to avoid future situations like #16192. This would fix #16047 in a simplistic manner and doesn't attempt to differentiate between different types of background updates.

This still would require us to do thinking about what old updates are now required, but I think it would fix the issue where we have to continually create background jobs due to dependencies.

## TODO

* [x] Enforce that `ordering` is filled in (non-nullable).
* [x] ~~Maybe try to fix that `BACKGROUND_UPDATES_COMPAT_VERSION < last full schema dump`?~~
* [x] Documentation.