Feeds: Always return a valid timestamp for the Last-Modified header of comment or post feeds.
Fixes bug where an invalid Last-Modified value would be returned in feed requests for sites that had 0 items to return. Comment or post feeds will now return the current timestamp as the Last-Modified header value. Example: a request for the comments feed for a site without any comments.
Replaced use of the local static variable $cache_lastcommentmodified to store the modified date in get_lastcommentmodified() with the Object Cache API. The get_lastcommentmodified() function returns early if there is a cached value and returns false if there where no comments found. Introduced _clear_modified_cache_on_transition_comment_status() to flush the lastcommentmodified cache key when a comment enters or leaves approval status. In get_lastpostmodified() return early if there is a cached value and return false if there are no posts found.
Props swissspidy, rachelbaker, dllh, leobaiano.
Fixes #38027.