Sync stable to main

We should be careful that stable will not run behind main more than necessary.

It seems like we never synced stable to main when Discourse 3.0 was released (which is a typical moment to sync them, since Discourse core is also synced then (Discourse 3.0 is 100% equal to Discourse 3.1.0 beta 1).

We should test whether the current main branch passes all tests on Discourse 3.0.

If so, we should make stable point to the current commit on main and keep it synchronized until we have an actual new feature that is not compatible with Discourse 3.0.

If not, we should see if there are any large issues and fix them if the effort is relatively low