Update Java version to 21 (#252)

Author: JCRPaquin

build.gradle.kts

@@ -4,3 +4,12 @@
  * This is a general purpose Gradle build.
  * To learn more about Gradle by exploring our Samples at https://docs.gradle.org/8.2.1/samples
  */
+plugins {
+    java
+}
+
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(21))
+    }
+}