mail/thunderbird: update to 137.0 (rc1)

Release Notes (soon):

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

Mention the big upgrade and the new thunderbird-esr port in UPDATING.

(cherry picked from commit 7eb19a67e08cec39e733e1ffd8c63898bdfa7992)