Update for Mailcow 2025-03

[dorianim]

Mailcow changed the authentication system in their March 2025 update, which likely makes some major changes necessary.

Warning

Existing users should switch to the legacy branch of Mailcow until this package is updated:
./update.sh --legacy

For reference: https://ask.linuxmuster.net/t/mailcow-update-maerz-2025/11592