mergemaster: postpone removal to FreeBSD 15

Mergemaster has been deprecated for quite some time, but was not
removed prior to FreeBSD 14.0. Update the deprecation notice in the man
page to reflect this.

PR: 274967
Reported by: naddy
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 73cce7d8886bc59284c757decb49deba046e73fc)