Fix DataGen in Neoforge 1.21.1

neoforge/build.gradle

@@ -24,6 +24,11 @@ neoForge {
         }
         data {
             data()
+
+            // DataGen can be run by - "./gradlew :neoforge:runData" in Terminal.
+            // Specify the modid for data generation, where to output the resulting resource, and where to look for existing resources.
+            programArguments.addAll '--mod', project.mod_id, '--all', '--output', file('src/generated/resources/').getAbsolutePath(), '--existing', file('src/main/resources/').getAbsolutePath()
+
         }
         server {
             server()