rollback sourceCompatibility since it's not providing new lang feature as expected

Author: ZZZank

build.gradle

@@ -10,10 +10,6 @@ base {
 version = project.mod_version
 group = project.maven_group
 
-java {
-    sourceCompatibility = JavaVersion.VERSION_21
-}
-
 loom {
     // use this if you are using the official mojang mappings
     // and want loom to stop warning you about their license
@@ -131,7 +127,7 @@ 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 = 21
+    sourceCompatibility = 16
 
     // 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.