Update autopkgtests to support dovecot 2.4

I also added a lintian override for shared-library-lacks-prerequisites, which is a false-positive in this case.

Unless you object or upload first, I'm going to NMU these changes on 2025-05-08 in order to ensure that dovecot related packages can migrate to trixie in a timely manner (per release team bug #1100698)