Change the 'recommended for external cache invalidation' set by the Purge module

A side effect of this implementation is that the extra max-age options added by the Purger module are now also available on the s-max-age fields we add, making ✨ Allow choosing more fine-grained max-ages Needs review less of an issue if the Purge module is installed.

Automatically closed - issue fixed for 2 weeks with no activity.