Warn users about per-table prefix in multisite during requirements hook

Adding a trigger_error() should be easy, just check for it being an array.

That said, that would result in dozens/hundreds of those deprecation messages for anyone who actually has that, because if it happens, it is a global sitewide thing that won't be easy to get rid of, you can't just update some code but likely have to rearchitecture your site. Assuming someone actually managed to make this work reliable, which I very much doubt, which is why we deprecated it in the first place.

Given that, not quite sure if a @trigger_error() is the right approach here. Maybe a requirements hook similar to the php version would make more sense?

Per-table prefix is theoretically deprecated since 8.2.0, but not enforced practically. That would be done by #3106531: Notify in Status Report that per-table database prefixes are no longer supported, and will throw errors in Drupal 10.0, which is now postponed on this issue.

I think about warning message at status page
It could be done in this issue for 9.2
To unblock #3106531: Notify in Status Report that per-table database prefixes are no longer supported, and will throw errors in Drupal 10.0 for 9.3.x

In #3106531-103: Notify in Status Report that per-table database prefixes are no longer supported, and will throw errors in Drupal 10.0 and #3106531-109: Notify in Status Report that per-table database prefixes are no longer supported, and will throw errors in Drupal 10.0 the release manager @catch made the decision that there will be no trigger_error() for the deprecation of per-table prefixing. Therefore having a seperate issue for adding a hook_requirements() prior to 9.3 and adding a trigger_error() in another is no longer necessary. Therefore closing this issue.

Changed to MR workflow and pushed a first commit taking hunks from current patch at #3106531: Notify in Status Report that per-table database prefixes are no longer supported, and will throw errors in Drupal 10.0

Testbot is not happy.

Looks like we cannot do the hook_requirements check in isolation. UpdatePathTestBase does for instance write a $database entry in settings.php as part of its setup; that has a 'prefix' as an array, and test update fails on the new check. So one way or another we need to adjust also other code. But that's what #3106531: Notify in Status Report that per-table database prefixes are no longer supported, and will throw errors in Drupal 10.0 already does in a more complete way and I am not going to reinvent that here.