Github actions does not run ubuntu-20.04 anymore due to EOL. Update the
20.04 entries to use 24.04.
cpp-coveralls needs an older version of python3 due to pkgutil.ImpImporter
having disappeared in more recent python versions. Therefore, leave
test-coveralls at 22.04.
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>