Add Dovecot 2.4 support to Puppet

Our Puppet-managed deployment of Dovecot isn't compatible with trixie, as startup fails with:

dovecot[9560]: doveconf: Fatal: Error in configuration file /etc/dovecot/dovecot.conf line 2: The first setting must be dovecot_config_version

Several non-trivial, non-backwards-compatible changes are required in our configuration files to support trixie:

• https://github.com/dovecot/core/releases/tag/2.4.0
• https://doc.dovecot.org/2.4.0/installation/upgrade/2.3-to-2.4.html