heading for release

I think we're heading for 2.53.1, as we've come to a pause in fixing
things.  Right now I am contemplating only:

  update NEWS
  try to make some progress on the dune/opam front
  declare an rc1 and ask unison-users to test

It would be helpful for people to try to build and run the tip of
master, and file bugs or write here if issues.

It would be especially helpful if anyone can build on Windows, as github
CI seems troubled.  (It's possible this release won't have windows
binaries.)