[policy] Remove update hooks added until 10.3 from Drupal 11

Moving to needs review.

Assuming we go ahead with this plan, we should open a postponed issue and remove the old database updates as close as possible to the 10.3 beta, to minimize the chances of having to regenerate database dumps multiple times since that's still time consuming and error prone. On the other hand if we're able to get #3403649: Rework database update tests so we don't have to ship database dumps in git done it might then be easier to remove updates on a rolling basis.

This could use a +1 from a release manager who's not me still I think, tagging for that.

There are three reasons to remove older update functions:

1. To encourage sites to update contributed modules to 11.x-compatible versions before updating core to 11.x. i.e. sites on 10.3.x with updated contrib modules are much more likely to update to 11.0.x successfully than sites on 10.0.0 trying to jump all the way to 11.x and updating their contrib modules at the same time.

Sites on 10.0 may not be able to update contrib modules to 11.x compatible versions at all (if they rely on APIs added in 10.1 or 10.2), and they also may struggle to composer update contrib + core all the way from 10.0 to 11.0, because updating lots of composer dependencies at the same time gets very, very complicated.

2. To minimize the number of (core and contrib) update functions that can potentially have to run in the same update, for example if a site tried to update all the way from 10.0.0 to 11.6.0 (which would represent 4-5 years of core development) there could be tens of updates that need to run, config that gets updated multiple times for different changes etc. The more updates can potentially run, the more likely it is that we end up with dependency issues where update X has to run before update Y but after update Z and similar.

3. To reduce the matrix of update paths that we don't have test coverage for. e.g. ideally we'd have test coverage for 10.3-11.0, 10.4-11.0, 10.5-11.0, 10.3-11.1, 10.4-11.1, 10.5-11.1, 11.0-11.1 etc. but we don't have anything like that and can't add it with the current update test infrastructure.

I think that #1 is about the same with 10-11 as it was for 9-10. But #2 and #3 will actually be worse with two active branches, since we'll have more total minor releases to juggle, more updates paths to potentially backport between major versions (or not), and more potential paths to for people to take between different combinations of versions.

Or tl;dr, we've said that 10.x will be supported until Drupal 12.0, but we didn't say that Drupal 10.0 will be supported until 12.0.

fwiw I discussed #7 briefly with Gábor in slack and we realised that removing updates from 10.0-10.3 from 11.x doesn't preclude updating from Drupal >= 10.3 to 12.0.

That will depend on the functions added from 10.4/11.0 onwards and when those get removed. e.g. there could be a situation where you can update from 10.6 and 11.4 to 12.0, but not from 10.5 and 11.3. Or it could be that you can update only from 11.4+ to 12.0 - it all depends what gets backported to Drupal 10 minor releases, and what the cut-off point is. This doesn't automatically mean we'll support a Drupal 10-12 update path, but it adds an extra thing to think about in #2942096: [policy, no patch] Remove old update hooks prior to each major version (Drupal 10 and later) and is unaffected by this issue.

What this issue will do is prevent direct updates from Drupal 9.5-10.2 to Drupal 11.

I've updated #2942096: [policy, no patch] Remove old update hooks prior to each major version (Drupal 10 and later) quoting/linking some of the discussion here. Now that this has had an additional +1 from another release manager, going to mark it fixed. Have also opened #3416928: [Meta] Remove updates added up until 10.3.0 from Drupal 11 for the implementation issue.