build: Set JVM source and target version

build.gradle.kts

@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+
 plugins {
     alias(libs.plugins.kotlin)
     alias(libs.plugins.ktor)
@@ -35,6 +37,17 @@ ktor {
     }
 }
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_21
+    targetCompatibility = JavaVersion.VERSION_21
+}
+
+kotlin {
+    compilerOptions {
+        jvmTarget = JvmTarget.JVM_21
+    }
+}
+
 repositories {
     mavenCentral()
     google()