15.1: Improve upgrade instructions
- Upgrading 14.x pkgbase to 15 is not supported
- Upgrading pkg to upgrade the base system is not required
- Third party kmods must only be upgraded if using them
Reported by: markj (efi case varies, bectl is zfs only)
Co-authored-by: Alexander Ziaee <ziaee@FreeBSD.org>
Differential Revision: https://reviews.freebsd.org/D57606