mail/vexim: update to latest (cdfb0df) git snaphot to incorporate security fixes

See also: https://github.com/vexim/vexim2/commit/cdfb0dfd4130e1f3c1690c2c7e256b580f0d6424

MFH: 2022Q1
Submitted by: VVD via irc
Sponsored by: Netzkommune GmbH

(cherry picked from commit 32b45d01948b720e1bcafba2ee9bd183cbc9baad)