Deprecate InlineFormInterface::getEntityTypeLabels()

Updated the proposed resolution because I don't think we can change InlineEntityFormBase::getEntityTypeLabels() to use the entity label formatters yet. If we did, then no InlineFormInterface::getEntityTypeLabels() functions would be getting executed at all, breaking backward compatibility.

Noted that a change record should be created.

The MR covers items 2, 3, and 4 from the proposed resolution. It even has a deprecation test, per Drupal's deprecation policy. The majority of the changes resulted from the removal of NodeInlineForm::getEntityTypeLabels() because Core tried to get away from using the word "node" on the front end in favor of "content item." So all of the assertions that were looking for the word "node" in the UI had to be updated.

I should have searched for existing issues. Shame on me. See #3051925: deal with TODO for 1850080 → .

This needs a rebase / reroll now that 📌 Remove hardcoded word 'entities' in EntityInlineForm::getEntityTypeLabels() Fixed is committed.

Thanks,
-Derek

This was my first experience using the GitLab conflict resolver tool. At least in this case it made the resolution quick and easy, but that's at least partly because this MR contained the exact same changes that caused the conflict.

I also took the time to resolve the PHPCS issues that were introduced by the patch. Although there are now two more issues, but that's due to me using placeholder text for the change record URL that needs to be added to the deprecated function. I haven't drafted a CR yet because there hasn't been any consensus about the change.

Great, thanks! A few updates to the summary. +1 to the plan. Feel free to draft a CR for this so we can get the link right and see a fully green pipeline here.

Thanks again,
-Derek