[policy, no patch] Deprecate and remove the Block UI

+1 from my point of view as a framework maintainer. Leaving tag to give other FMs the change to respond too

This will have a hard dependency on #3555047: Add a way to ship block placement statically (will be useful when block UI is removed) which has an outline of an idea but no code yet. I think once we have that, then we have a way to run a site without a layout builder (someone could do the same with blocks in their front end theme as we do in core for the admin theme) and we can point people to the various contrib solutions that provide a UI to control page-level layout amongst other things.

I'm not sure if our final state should be that core doesn't have any layout solution in it, but I think removing block module will make it easier to think about how to move a method back in, and that method would never be block module again. Leaving the FM/RM tags on partly because I think I proposed this in the first place at some point.

Mentioned this in #3555047-11: Add a way to ship block placement statically (will be useful when block UI is removed), but we might need to keep block config entities in order to account for plugin dependencies. Or managing dependencies would need to be part of whatever the replacement is.