feat(postfix): implement virtual mailbox configuration for Dovecot LMTP

Updates the Postfix configuration to correctly handle virtual domains when Dovecot LMTP is enabled, moving away from local system delivery settings.

- Removes `postfix_mail_domain` from `postfix_mydestination` to prevent conflicts with virtual domain handling.
- Updates `main.cf` to set `virtual_transport`, `virtual_mailbox_domains`, and `virtual_mailbox_maps` instead of `mailbox_transport`.
- Adds a new template `virtual_mailbox_maps.j2` to authorize specific users defined in `dovecot_users`.
- Adds tasks to generate the virtual mailbox map file and run `postmap` upon changes.

templates/virtual_mailbox_maps.j2

@@ -0,0 +1,8 @@
+# Virtual mailbox maps for Postfix
+# Ansible managed: {{ ansible_managed }}
+# Format: user@domain   OK
+{% if dovecot_users is defined and dovecot_users | length > 0 %}
+{% for user in dovecot_users %}
+{{ user.name }}@{{ postfix_mail_domain }}   OK
+{% endfor %}
+{% endif %}