mfc-candidates: use stable/14 as default MFC-to branch

This tool is typically invoked from within a working tree containing the
desired MFC target branch, which is detected automatically. It also has
a default for when it is invoked from elsewhere. Switch the default to
the most recent branch, stable/14.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 76a8625bc71da3fa63eef5abb4782553d2588f5c)