System move procedural hooks and mark as converted

This was already RTBC before it was split up. I’m busy this weekend, but hope to look on Monday to confirm this is still ready.

Reviewed the changes and they all make sense.

the system.theme.inc MAY be out of scope but personally think it's fine here. Will see what the committer thinks .

Left a couple of questions on the MR - thanks

Let's postpone this for a bit on 📌 Handle module preprocess functions Active

blocker is in

I'd suggest we keep this postponed on 📌 Convert template preprocess in system.module Active