ci: Update Fedora CI image to Fedora 39

Fedora 37 is out of support so, as per our policy, update the CI image to the oldest still-supported release, which is 39.

Update the mingw CI image too, as it’s built on top of the Fedora one.

Update the supported platforms documentation (and fix the Debian version listed there to match what’s currently in CI, which is up to date).

Signed-off-by: Philip Withnall pwithnall@gnome.org