mail/thunderbird: switch to bundled botan2

Our botan2 port is going to expire soon (2025-01-31, see eb1217e8e9).
Switching to botan3 is not a trivial change (e.g. botan3 requires C++20),
so I'll rather wait for upstream to make this change first (see
https://bugzilla.mozilla.org/show_bug.cgi?id=1848104). To avoid
breakages and holdups, just switch to the bundled botan2.