Azure CI: use Ubuntu-20.04 image by default
    
    Ubuntu 20.04 image is available in Azure Pipelines, migrate to it.
    https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-README.md
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>