Removes `testenv` annotations and related code. These annotations originally let you skip x-pack snippet tests in the docs. However, that's no longer possible.
Relates to #79309, #31619
Add an _upgrade endpoint to bulk upgrade transforms. _upgrade rewrites all transforms and its
artifacts into the latest format to the latest storage(index). If all transforms are upgraded old
indices and outdated documents get deleted. Using the dry_run option it is possible to check if
upgrades are necessary without applying changes.
