Specify correct supported puppet version, 8.x only

Moving to pdk template 3.5.1 broke ruby 2.7 support, and by
extension puppet 7.x.

Author: tvpartytonight

metadata.json

@@ -18,7 +18,7 @@
   "requirements": [
     {
       "name": "puppet",
-      "version_requirement": ">= 6.0.0 < 9.0.0"
+      "version_requirement": ">= 8.0.0 < 9.0.0"
     }
   ],
   "pdk-version": "3.5.1",