releases: Expand rel-latest macro

There are a few places where we compare a RELEASE to it's STABLE branch,
or a RELEASE to it's previous release. Use macros so we can bump these
instead of bumping the numbers constantly.

Outstanding:

• I don't know what to do with rel-latest-previous since we're at a .0

currently, it should be whatever the reader should update to before they
update to 15.0. The jails chapter would benefit greatly from that.

• The syntax for escaping to use a macro inside of a code block is

still I can't figure it out... but other than that it's working well!

Fixes: ff71b333bffa (releases: define rel-latest macro)
Reviewed by: mhorne
Suggested by: Jan Brankamp <crest@rlwinm.de>
Differential Revision: https://reviews.freebsd.org/D56104