Upgrade Thunderbird to 60.7.2+

https://www.mozilla.org/en-US/security/known-vulnerabilities/thunderbird/#thunderbird60.7.2

Feature Branch: https://salsa.debian.org/tails-team/tails/merge_requests/31

Related issues

Blocks #16209

Original created by @intrigeri on 16834 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information