Use Python 3.13 in generated cog.yaml (#2376)
Python 3.11 is now two versions old and 3.13 has been out for over a
year at this point.
This commit also removes the `fast: true` flag which is unlikely
to be supported going forward.
Co-authored-by: Will Sackfield <sackfield@replicate.com>
Co-authored-by: Michael Dwan <m@dwan.io>
