Change postfix configuration to use the latest compatibility level

During work for updating config files up to bookworm (#40723 (comment 3177689)) we saw that the main.cf that we configure everywhere via the puppet postfix module is using compat level 2 everwhere, which not the oldest but not the most recent either -- starting with postfix 3.6 the compatibility level has a value of the latest major version supported.

We'd like to move our configuration to the most recent compatibility_level (for bookworm that's 3.7; for trixie that's 3.10)