Add cache invalidation when updating a term, example: create a category,...

Add cache invalidation when updating a term, example: create a category, assign it to a post, edit the category. Currently, the post's term cache is not updated. When updating terms in a given taxonomy, invalidate the object term caches linked to that taxonomy. 

Introduce `get_taxonomy_last_changed()`, `set_taxonomy_last_changed()`, and `post_taxonomy_is_fresh()`.

`post_taxonomy_is_fresh()` is only called in `get_object_term_cache()` - at which point the taxonomy's `last_changed` value is checked against the post's `{$taxonomy}_last_changed` value.

`set_taxonomy_last_changed()` is called whenever directory database queries are made that insert new terms or affect existing terms.

Fixes #22526.


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