Update dependency puppet/postfix to v6

Author: renovate[bot]

Puppetfile

@@ -86,7 +86,7 @@ mod 'puppet/mosquitto', '2.2.0'
 mod 'puppet/nfs', '3.0.0'
 mod 'puppet/nginx', '7.0.1'
 mod 'puppet/openldap', '8.1.0'
-mod 'puppet/postfix',  '5.1.0'
+mod 'puppet/postfix',  '6.0.0'
 mod 'puppet/prometheus', '16.5.0'
 mod 'puppet/python', '8.0.0'
 mod 'puppet/quadlets', '1.2.0'