Set kotlin jvmToolchain for jvm modules.

Closes #13686

Fixes #13523

core-util-jvm/build.gradle.kts

@@ -4,6 +4,7 @@
  */
 
 val signalJavaVersion: JavaVersion by rootProject.extra
+val signalKotlinJvmTarget: String by rootProject.extra
 
 plugins {
   id("java-library")
@@ -16,6 +17,12 @@ java {
   targetCompatibility = signalJavaVersion
 }
 
+kotlin {
+  jvmToolchain {
+    languageVersion = JavaLanguageVersion.of(signalKotlinJvmTarget)
+  }
+}
+
 dependencies {
   implementation(libs.kotlin.reflect)