Switch to another Puppet module to manage Postfix

Originally created by @intrigeri on #15511 (Redmine)

https://gitlab.com/shared-puppet-modules-group/postfix is deprecated.

One option is to switch to https://github.com/camptocamp/puppet-postfix but quite some features are missing e.g.
https://gitlab.com/shared-puppet-modules-group/postfix/milestones/1.

Parent Task: tails/sysadmin#15500

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