build(gradle): set kotlin jvmTarget to JVM_21

Java sourceCompatibility/targetCompatibility were already 21, but
the Kotlin compiler defaulted to JVM 17 bytecode, producing a
toolchain skew warning on every build. Align the Kotlin target to
match the Java target.

app/build.gradle.kts

@@ -2,6 +2,7 @@ import com.android.build.api.artifact.SingleArtifact
 import java.io.ByteArrayOutputStream
 import javax.inject.Inject
 import org.gradle.process.ExecOperations
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 
 plugins {
     alias(libs.plugins.android.application)
@@ -65,6 +66,12 @@ android {
     }
 }
 
+kotlin {
+    compilerOptions {
+        jvmTarget.set(JvmTarget.JVM_21)
+    }
+}
+
 dependencies {
     compileOnly(project(":stub"))
     compileOnly(libs.annotation)