Remove block.module dependency from Layout Builder

Rebased the MR to latest 11.x Head and fixed Merge Conflicts.

Seems to have failures. Also still need a change record.

https://git.drupalcode.org/project/drupal/-/merge_requests/778 had conflicts in almost every file + the changes to theme hooks and preprocess made this impossible to rebase. I've started a fresh branch here https://git.drupalcode.org/project/drupal/-/merge_requests/12313

I don't think moving the template_preprocess_block and preprocessBlock functions will cause any issues.

acbramley → closed merge request !778

acbramley → changed the visibility of the branch 3003610-block to hidden.

Had a quick look at how to deprecate the block template instead of removing it but as per #25 it looks like it's targetting theme hooks, not templates? Can we keep the block theme hook in block module as well as system?

I've tested having the theme hook in both modules and I don't think it's going to work because the theme registry does this:
$cache = NestedArray::mergeDeep($cache, $result);
Which merges both definitions together, meaning the system definition will override the block definition but keep the deprecation message.

I also tested just keeping the template around in block module without the theme hook and tests still pass when other templates use @block/block.html.twig but then we have no way of throwing the deprecation.