gh-125260: Change the default ``gzip.compress()`` mtime to 0 (#125261)

This follows GNU gzip, which defaults to using 0 as the mtime
for compressing stdin, where no file mtime is involved.

This makes the output of gzip.compress() deterministic by default,
greatly helping reproducible builds.

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>