The installation note does not contain installation instructions, and
has not since FreeBSD 6.0. Rename it to the upgrading note to avoid
confusion with the installation guide. Consolidate all release upgrading
information here.
Rewrite the introduction to the release note to separate these out more.
Remove information that is not needed to introduce this release.
The release installation notes do not actually contain instructions on
installing FreeBSD, so rename it to the Upgrading Note.
I used Vladlen's previous diff where he invented the pkgbase info, boot
loader info, and the concept of numbering, which is the icing that ties
this together. His diff is located at https://reviews.freebsd.org/D57421
Co-authored-by: Vladlen Popolitov <vladlen@FreeBSD.org>