Move the cache of post parent IDs from the dedicated group `post_parents` to `posts`. This maintains backward compatibility for clearing all post object related data by calling `wp_cache_flush_group( 'posts' )`.
Post parent IDs are now cached with with the prefix `post_parent:` in the `posts` group.
Follow up to [56763].
Props spacedmonkey, joemcgill, peterwilsoncc.
See #59188.
git-svn-id: https://develop.svn.wordpress.org/trunk@56925 602fd350-edb4-49c9-b593-d223f7449a82