Release notes: fix Thunderbird version

Note that the upstream 91.5.0 release notes prominently say "For more on all the
new features in Thunderbird 91, see New in Thunderbird 91.0", and link to
https://support.mozilla.org/en-US/kb/new-thunderbird-91/, which seems much more
informative than
https://www.thunderbird.net/en-US/thunderbird/91.0/releasenotes/ for our
target audience.

So our users will still have a chance to get information about the 78 →
91 changes.