mail/thunderbird: update to 137.0 (rc2)

Release Notes:

https://www.thunderbird.net/en-US/thunderbird/137.0/releasenotes/

While here, re-check system icu usability - still broken, so just
update the comment.

(cherry picked from commit e78f6a1168991be92351f81f5a27e7639e900132)