ssh: remove pre- and post-merge update steps

We no longer use the pre- and post-merge scripts to strip/add RCS tags.
The tags have been removed from main, but persist on older branches.

While here renumber the steps in the update documentation using a more
conventional scheme.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36904

(cherry picked from commit fca7ac55f8db80b5250bd80a83e1368fe0c39ae5)