set sourceCompatibility to 21

build.gradle

@@ -11,7 +11,7 @@ version = project.mod_version
 group = project.maven_group
 
 java {
-    sourceCompatibility = targetCompatibility = JavaVersion.VERSION_1_8
+    sourceCompatibility = JavaVersion.VERSION_21
 }
 
 loom {
@@ -131,15 +131,12 @@ tasks.withType(JavaCompile) {
     // see http://yodaconditions.net/blog/fix-for-java-file-encoding-problems-with-gradle.html
     // If Javadoc is generated, this must be specified in that task too.
     options.encoding = "UTF-8"
-    sourceCompatibility = 16
+    sourceCompatibility = 21
 
     // The Minecraft launcher currently installs Java 8 for users, so your mod probably wants to target Java 8 too
     // JDK 9 introduced a new way of specifying this that will make sure no newer classes or methods are used.
     // We'll use that if it's available, but otherwise we'll use the older option.
-    def targetVersion = 8
-    if (JavaVersion.current().isJava9Compatible()) {
-        options.release = targetVersion
-    }
+    options.release = 8
 }
 
 java {