Update GlobalConfiguration.mustache (#11359)

[csharp-netcore] corrected visibility of GlobalConfiguration mustache

modules/openapi-generator/src/main/resources/csharp-netcore/GlobalConfiguration.mustache

@@ -12,7 +12,7 @@ namespace {{packageName}}.Client
     /// A customized implementation via partial class may reside in another file and may
     /// be excluded from automatic generation via a .openapi-generator-ignore file.
     /// </remarks>
-    public partial class GlobalConfiguration : Configuration
+    {{>visibility}} partial class GlobalConfiguration : Configuration
     {
         #region Private Members
 
@@ -56,4 +56,4 @@ namespace {{packageName}}.Client
             }
         }
     }
-}
+}