Preparing for Discourse's upgrade to Ember 5

The next stable is scheduled for 30th January, and AFAIK we’re still on-track for that.

Ember 5 will certainly be the default in the 3.2 release. TBC on whether Ember 3 will still be available behind a flag. I think most likely yes, but we’ll aim to make it clear that it’s not a ‘supported’ configuration (e.g. via an admin warning).