As discussed during today's Services/Reading sync meeting we want to add content format versioning to Content-Type header, so we can select pages which need to get regenerated due to content format changes by MCS code easier. This will help aid in getting T136964 resolved faster, and future-proof the service.

Whenever we change the content format we should increment that version number.

See https://www.mediawiki.org/wiki/API_versioning#Content_format_stability_and_-negotiation