CI: bump to use F39
And drop the job to test the minimum meson version in the process. F39 ships with python 3.12 which causes this error with meson 0.56.0
proto/meson.build:17:0: ERROR: <ExternalProgram 'python3' -> ['/usr/bin/python3']> is not a valid python or it is missing setuptools
We could put the effort into setting up the CI to fix this, but we won't.