gradle: Build with JDK 21

Checkstyle was already requiring JDK 21 and now about libraries need it too

Use the kotlin extension method as it will also configure it for java code

Ref: https://kotlinlang.org/docs/gradle-configure-project.html#gradle-java-toolchains-support

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>

app/build.gradle.kts

@@ -20,13 +20,8 @@ val gitWorkingBranch = providers.exec {
     commandLine("git", "rev-parse", "--abbrev-ref", "HEAD")
 }.standardOutput.asText.map { it.trim() }
 
-java {
-    toolchain {
-        languageVersion = JavaLanguageVersion.of(17)
-    }
-}
-
 kotlin {
+    jvmToolchain(21)
     compilerOptions {
         // TODO: Drop annotation default target when it is stable
         freeCompilerArgs.addAll(
@@ -137,13 +132,6 @@ ksp {
 // Custom dependency configuration for ktlint
 val ktlint by configurations.creating
 
-// https://checkstyle.org/#JRE_and_JDK
-tasks.withType<Checkstyle>().configureEach {
-    javaLauncher = javaToolchains.launcherFor {
-        languageVersion = JavaLanguageVersion.of(21)
-    }
-}
-
 checkstyle {
     configDirectory = rootProject.file("checkstyle")
     isIgnoreFailures = false