fix(client): support kotlin 1.8 runtime

buildSrc/build.gradle.kts

@@ -1,6 +1,6 @@
 plugins {
     `kotlin-dsl`
-    kotlin("jvm") version "2.1.10"
+    kotlin("jvm") version "1.9.20"
     id("com.vanniktech.maven.publish") version "0.28.0"
 }
 
@@ -11,6 +11,6 @@ repositories {
 
 dependencies {
     implementation("com.diffplug.spotless:spotless-plugin-gradle:7.0.2")
-    implementation("org.jetbrains.kotlin:kotlin-gradle-plugin:2.1.10")
+    implementation("org.jetbrains.kotlin:kotlin-gradle-plugin:1.9.20")
     implementation("com.vanniktech:gradle-maven-publish-plugin:0.28.0")
 }

buildSrc/src/main/kotlin/openlayer.java.gradle.kts

@@ -23,6 +23,9 @@ java {
     toolchain {
         languageVersion.set(JavaLanguageVersion.of(17))
     }
+
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
 }
 
 tasks.withType<JavaCompile>().configureEach {

buildSrc/src/main/kotlin/openlayer.kotlin.gradle.kts

@@ -1,6 +1,6 @@
 import com.diffplug.gradle.spotless.SpotlessExtension
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
-import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 
 plugins {
     id("openlayer.java")
@@ -11,25 +11,26 @@ kotlin {
     jvmToolchain {
         languageVersion.set(JavaLanguageVersion.of(17))
     }
-}
-
-configure<SpotlessExtension> {
-    kotlin {
-        ktfmt().kotlinlangStyle()
-        toggleOffOn()
-    }
-}
 
-tasks.withType<KotlinCompile>().configureEach {
     compilerOptions {
-        allWarningsAsErrors = true
         freeCompilerArgs = listOf(
             "-Xjvm-default=all",
             "-Xjdk-release=1.8",
             // Suppress deprecation warnings because we may still reference and test deprecated members.
-            "-Xsuppress-warning=DEPRECATION"
+            // TODO: Replace with `-Xsuppress-warning=DEPRECATION` once we use Kotlin compiler 2.1.0+.
+            "-nowarn",
         )
         jvmTarget.set(JvmTarget.JVM_1_8)
+        languageVersion.set(KotlinVersion.KOTLIN_1_8)
+        apiVersion.set(KotlinVersion.KOTLIN_1_8)
+        coreLibrariesVersion = "1.8.0"
+    }
+}
+
+configure<SpotlessExtension> {
+    kotlin {
+        ktfmt().kotlinlangStyle()
+        toggleOffOn()
     }
 }
 

openlayer-java-example/build.gradle.kts

@@ -1,16 +1,20 @@
 plugins {
-    id("openlayer.kotlin")
     id("java")
     application
 }
 
+repositories {
+    mavenCentral()
+}
+
 dependencies {
     implementation(project(":openlayer-java"))
 }
 
-tasks.withType<JavaCompile>().configureEach {
+java {
     // Allow using more modern APIs, like `List.of` and `Map.of`, in examples.
-    options.release.set(9)
+    sourceCompatibility = JavaVersion.VERSION_1_9
+    targetCompatibility = JavaVersion.VERSION_1_9
 }
 
 application {