gradle: Build with JDK 21

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

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

Author: theimpulson

app/build.gradle.kts

@@ -24,13 +24,8 @@ 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(
@@ -142,13 +137,6 @@ 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