try to fix kotlin cross tests
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 936a018..2b60521 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -662,7 +662,7 @@
         # kotlin cross test are failing -> see THRIFT-5879
         server_lang: ['java', 'go', 'rs', 'cpp']
         # we always use comma join as many client langs as possible, to reduce the number of jobs
-        client_lang: ['java', 'go,rs,cpp']
+        client_lang: ['java,kotlin', 'go,rs,cpp']
       fail-fast: false
     steps:
       - uses: actions/checkout@v6
diff --git a/lib/kotlin/cross-test-client/build.gradle.kts b/lib/kotlin/cross-test-client/build.gradle.kts
index 6f27932..8aa57b6 100644
--- a/lib/kotlin/cross-test-client/build.gradle.kts
+++ b/lib/kotlin/cross-test-client/build.gradle.kts
@@ -16,6 +16,8 @@
  * specific language governing permissions and limitations
  * under the License.
  */
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 plugins {
     kotlin("jvm")
@@ -48,6 +50,18 @@
     testImplementation("org.jetbrains.kotlin:kotlin-test-junit")
 }
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+}
+
+tasks.withType<KotlinCompile> {
+    compilerOptions {
+        jvmTarget = JvmTarget.JVM_1_8
+        freeCompilerArgs = listOf("-Xjdk-release=1.8")
+    }
+}
+
 val keyStore: String =
     file("$projectDir/../../java/src/crossTest/resources/.clientkeystore").canonicalPath
 val trustStore: String =