Drop puppet 4/5; Add Puppet 7

Signed-off-by: Christos Papageorgiou <christos.papageorgioy@gmail.com>

metadata.json

@@ -72,7 +72,7 @@
   "requirements": [
     {
       "name": "puppet",
-      "version_requirement": ">= 4.0.0 < 7.0.0"
+      "version_requirement": ">= 6.1.0 < 8.0.0"
     }
   ],
   "description": "Postfix Module for Puppet",