Simplify HtmlRestrictions and FundamentalCompatibilityConstraintValidator now that "forbidden tags" are deprecated

This should wait for #3228334: Refactor HTMLRestrictionsUtilities to a HtmlRestrictions value object to finish; that will make this simpler.

#3228334: Refactor HTMLRestrictionsUtilities to a HtmlRestrictions value object is in!

This issue will have to remove these two @todos. Let's see what fails.

Postponed on decision in #3272516: Deprecate FilterInterface::getHTMLRestrictions()' forbidden_tags functionality, potentially this will be a Closed (won't fix).

It also means that this is definitely not a stable blocker, per @catch's input at #3272516. So will remove this from #3238333: Roadmap to CKEditor 5 stable in Drupal 9.

#3272516: Deprecate FilterInterface::getHTMLRestrictions()' forbidden_tags functionality landed!

Was pointed here from #3280343: Audit of CKEditor 5 @todo items, realized that while we didn't need to implement this anymore thanks to #3272516: Deprecate FilterInterface::getHTMLRestrictions()' forbidden_tags functionality, we should still remove all @todos and detection logic!

The tricky thing is that #3272516: Deprecate FilterInterface::getHTMLRestrictions()' forbidden_tags functionality actually already removed most of this (but not all of it) in the 10.0.x branch. Which means that we need different patches for 9.5.x and 10.0.x.

OTOH, it's only deprecated in 9.5.x, so … does it really make sense to remove there? I think we should commit only the 10.0.x patch.

P.S.: Unfortunately there's also a bit of inconsistency in these files between 9.5.x and 10.0.x in HEAD … exactly because some of this was committed only to 10 and not 9. The after-applying-patches-diff-between-95-and-100.diff file shows the full set of differences, which proves that this makes everything consistent again. Reproduce that diff locally by doing git diff 9.5.x 10.0.x -- core/modules/ckeditor5/src after committing both patches locally.

I’ll tackle that tomorrow morning, unless somebody else gets to it before then!

#10 Ugh that wasn't around when I created after-applying-patches-diff-between-95-and-100.diff — I must've uploaded an old version of the D9 patch :/

The D10 patch here is effectively identical to that in #9.

Now what I wrote about git diff 9.5.x 10.0.x -- core/modules/ckeditor5/src should be true 😊😅

There's a leftover @todo pointing to this issue that should get addressed in both patches:
\Drupal\ckeditor5\HTMLRestrictions::unrestricted

    // @todo Refine in https://www.drupal.org/project/drupal/issues/3231336, including adding support for all operations.

Thanks for catching that! 👍

Excellent. Committed to 10.1.x which got cherry picked to 10.0.x, and 9.5.x got its own commit.

The fix should still be backported to 9.4.x.

Will provide a patch. Currently not cleanly cherry-pickable. So determining what the optimal cherry-pick order is …

#15 still applies cleanly to 9.4.x too! Test queued 👍

Agreed this looks fine for backport. Committed/pushed to 9.4.x, thanks!

Restoring previous status.