[DNF System Upgrade] Don't hardcode versions
    
    The current version of the page includes hardcoded, obsolete references
    to e.g. Fedora 28, 30, and 31, which can be confusing.
    
    Replace references with attributes, and add a new one, `{NEXTNEXTVER}`
    for the branched example.
    
    Signed-off-by: Michel Alexandre Salim <salimma@fedoraproject.org>