Add warning for post template removal

[annezazu]

While testing 6.3 beta 3, I noticed I could still remove the post template block and didn't receive a warning. This seems like a bit of an oversight as part of this PR: #51145 IMO we should have a warning for both, especially since if you remove the post template block, the experience is still broken:

no.warning.with.post.template.mov

[annezazu]

@tellthemachines in case you can spin up a quick PR to do the same for post template after your earlier work!

[tellthemachines]

Hmm the implementation in Gutenberg has diverged from the one in #51145 which went into 6.3, so if we want to fix this for 6.3 we either need to include #51841 or we'll have to do a custom patch for it directly in the release branch, and then fix it separately in Gutenberg. Looking at #51841 I think it should be safe to include and will definitely make things easier to not have two divergent implementations 😅