UPDATING: Remove old entries

Belatedly remove the entries older than the stable/11 branch point after
stable/13 was created. This should be done shortly after the branch, not
well after the branch point.

Document this policy in UPDATING, though other checklists should likely
be updated as well.

Sponsored by: Netflix
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37514