Upgrade to newer Fedora and Ubuntu

Ubuntu LTS is clearly the most used OS for us, followed by the latest Ubuntu release (when that's not an LTS) so we need to test those. In addition, testing on the next Ubuntu helps us ensure that our software will keep working on future Ubuntu releases.

I use Fedora, and maintain the *cangjie packages there, so having the CI run on the latest Fedora and on Rawhide (the next Fedora) is also a great help for me.

Using the virtual tags such as latest, we can make sure we always test against modern versions of those operating systems, rather than getting stuck on old versions because we forgot to bump a hardcoded number.

Fixes #1 (closed)