build(bulk-model-sync-gradle): set Java versions to 21 for test with Gradle

The tests execute the Gradle build for modelix.core which requires Java 21.
Java 21 is required by the build for the Gradle plugin `dev.petuska.npm.publish`.

Author: Oleksandr Dzhychko

bulk-model-sync-gradle/build.gradle.kts

@@ -3,6 +3,16 @@ plugins {
     `java-gradle-plugin`
 }
 
+tasks.test {
+    // The tests execute the Gradle build for modelix.core which requires Java 21.
+    // It requires Java 21 for the Gradle plugin `dev.petuska.npm.publish`.
+    javaLauncher.set(
+        javaToolchains.launcherFor {
+            languageVersion.set(JavaLanguageVersion.of(21))
+        },
+    )
+}
+
 dependencies {
     implementation(project(":model-client", "jvmRuntimeElements"))
     implementation(project(":bulk-model-sync-lib"))