(maint) Updates supported OS from Darwin to macOS

When metadata.json is consumed by Forge, it does not parse "Darwin"
as a version of OS X / macOS when filtering through operating systems.

Author: mhashizume

metadata.json

@@ -33,7 +33,7 @@
       "operatingsystem": "Fedora"
     },
     {
-      "operatingsystem": "Darwin"
+      "operatingsystem": "macOS"
     },
     {
       "operatingsystem": "SLES"