Drop RedHat 6; Add CentOS 7/8 support

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

metadata.json

@@ -43,7 +43,13 @@
     {
       "operatingsystem": "RedHat",
       "operatingsystemrelease": [
-        "6",
+        "7",
+        "8"
+      ]
+    },
+    {
+      "operatingsystem": "CentOS",
+      "operatingsystemrelease": [
         "7",
         "8"
       ]