[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>