Fix compilation errors after cherry-pick

Related to: #3957

The original commit use a `@Nullable` on some test configuration properties.
This branch does not have that import and there is really no reason in `@Nullable` in tests

* Remove `@Nullable` from the `ObservationTests`
* Fix `try-with-resource` warning in the `MessagingMessageListenerAdapter`
* Add explicit `jvmTarget` for Kotlin compiler in the `spring-kafka-docs` module

Author: artembilan

spring-kafka-docs/build.gradle

@@ -12,6 +12,20 @@ apply plugin: 'kotlin-spring'
 
 description = 'Spring Kafka Code Snippets For Docs'
 
+
+tasks.withType(JavaCompile).configureEach {
+    sourceCompatibility = JavaVersion.VERSION_17
+    options.encoding = 'UTF-8'
+}
+
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompilationTask).configureEach {
+    compilerOptions {
+        jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_17
+        javaParameters = true
+        allWarningsAsErrors = true
+    }
+}
+
 dependencies {
     implementation "org.springframework.boot:spring-boot-starter:$springBootVersion"
     implementation project (':spring-kafka')

spring-kafka/src/main/java/org/springframework/kafka/listener/adapter/MessagingMessageListenerAdapter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2024 the original author or authors.
+ * Copyright 2016-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -511,6 +511,7 @@ private RuntimeException checkAckArg(@Nullable Acknowledgment acknowledgment, Me
 	 * @param source the source data for the method invocation - e.g.
 	 * {@code o.s.messaging.Message<?>}; may be null
 	 */
+	@SuppressWarnings("try")
 	protected void handleResult(Object resultArg, Object request, @Nullable Acknowledgment acknowledgment,
 			Consumer<?, ?> consumer, @Nullable Message<?> source) {
 		final Observation observation = getCurrentObservation();

spring-kafka/src/test/java/org/springframework/kafka/support/micrometer/ObservationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022-2024 the original author or authors.
+ * Copyright 2022-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -803,6 +803,7 @@ AsyncFailureListener asyncFailureListener(SimpleTracer tracer) {
 		public TaskScheduler taskExecutor() {
 			return new ThreadPoolTaskScheduler();
 		}
+
 	}
 
 	public static class Listener {
@@ -888,11 +889,11 @@ public static class AsyncFailureListener {
 
 		final CountDownLatch asyncFailureLatch = new CountDownLatch(3);
 
-		volatile @Nullable SimpleSpan capturedSpanInListener;
+		volatile SimpleSpan capturedSpanInListener;
 
-		volatile @Nullable SimpleSpan capturedSpanInRetry;
+		volatile SimpleSpan capturedSpanInRetry;
 
-		volatile @Nullable SimpleSpan capturedSpanInDlt;
+		volatile SimpleSpan capturedSpanInDlt;
 
 		private final SimpleTracer tracer;
 
@@ -932,6 +933,7 @@ void handleDlt(ConsumerRecord<Integer, String> record, Exception exception) {
 			this.capturedSpanInDlt = this.tracer.currentSpan();
 			this.asyncFailureLatch.countDown();
 		}
+
 	}
 
 }