gh-84559: Change the multiprocessing start method default to `forkserver` (GH-101556)

Change the default multiprocessing start method away from fork to forkserver or spawn on the remaining platforms where it was fork.  See the issue for context.  This makes the default far more thread safe (other than for people spawning threads at import time... - don't do that!).

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>