First-draft release notes for 14.5.

As usual, the release notes for older branches will be made by cutting
these down, but put them up for community review first.

Due to the out-of-cycle release of 14.4, there are a number of commits
that appeared in 14.4 that are not yet shipped in the earlier branches.
This draft repeats those release note entries for convenience in
preparing the older-branch notes later.  They'll be stripped out of
the 14.5 section after that's done.