mail/thunderbird: update to 91.2.0

Release Notes:
  https://www.thunderbird.net/en-US/thunderbird/91.2.0/releasenotes/

This brings in Thunderbird 91esr:
- the calendar is not optional anymore
- gtk2 is not directly required anymore
- integrated PDF viewer
- and much more: https://support.mozilla.org/en-US/kb/new-thunderbird-91