[CI] comment out EL6 support as EL6 is EOL
[CI] comment out EL6 support as EL6 is EOL

And CI builds are already failing due to package repositories.
OpenISS/OpenISS,OpenISS/OpenISS,OpenISS/OpenISS,OpenISS/OpenISS,OpenISS/OpenISS,OpenISS/OpenISS,OpenISS/OpenISS