Fix an old obvious failure on my part

The 'Changes' and 'Upgrading' links are 6.6/6.5 general, not 6.6.1/6.5.2
specific

6.4 has passed its EOL date (2021-03), so it should not be listed in the
top release table any longer.

Fixes: #94

Thank you to Martin

rebuilt on commit.

also rename duplicate links

People tell me our home page was confusing, maybe this helps at least a
bit. But I guess the main reason will be the shift towards shorter
attention spans.