15.1: Installation note -> Upgrading note

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.

Reviewed by: carlavilla (previous), cperciva
Discussed with: cperciva, dch, emaste, fuz, michaelo
Co-authored-by: Vladlen Popolitov <vladlen@FreeBSD.org>
Differential Revision: https://reviews.freebsd.org/D57517