Set the core to use Java version Ω

build.gradle

@@ -630,14 +630,17 @@ allprojects { p ->
 
     tasks.withType(JavaCompile) {
         if (isMinecraftSubProject) {
-            options.release = rootProject.java_version as Integer
+            options.release = rootProject.java_version as Integer // Neoforge complains without this
             options.compilerArgs += ["-Xplugin:Manifold"]
-        } else {
-            options.release = 8; // Core & Api should use Java 8 no matter what
-            //options.release = rootProject.java_version as Integer // But if you want to test some stuff, then this can be enabled
         }
         options.encoding = "UTF-8"
     }
+    
+    // Sets the project's actual Java version (it's recommended to use this over the `options.release` method above)
+    java {
+        sourceCompatibility = rootProject.java_version
+        targetCompatibility = rootProject.java_version
+    }
 
     java {
         withSourcesJar()