MFH: r533707 r534066

update thunderbird to 68.8.0 (rc1)

Announcement (soon):

https://www.thunderbird.net/en-US/thunderbird/68.8.0/releasenotes/

update thunderbird to 68.8.0 (rc2)

Announcement (soon):

https://www.thunderbird.net/en-US/thunderbird/68.8.0/releasenotes/

Approved by: ports-secteam (blanket: web browser alike)