Fix serialization compiler warnings with Java 18

As of Java 18, the serial lint warning in javac has been expanded to
check for class fields that are not marked as `Serializable`.
See https://www.oracle.com/java/technologies/javase/18all-relnotes.html#JDK-8202056

In the Spring Framework codebase, this can happen with `Map`, `Set` or
`List` attributes which are often assigned with an unmodifiable
implementation variant. Such implementations are `Serializable` but
cannot be used as field types.

This commit ensures that the following changes are applied:
* fields are marked as transient if they can't be serialized
* classes are marked as `Serializable` if this was missing
* `@SuppressWarnings("serial")` is applied where relevant

spring-messaging/src/main/java/org/springframework/messaging/MessageHeaders.java

@@ -115,7 +115,7 @@ public class MessageHeaders implements Map<String, Object>, Serializable {
 	@Nullable
 	private static volatile IdGenerator idGenerator;
 
-
+	@SuppressWarnings("serial")
 	private final Map<String, Object> headers;
 
 

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompHeaders.java

@@ -104,6 +104,7 @@ public class StompHeaders implements MultiValueMap<String, String>, Serializable
 	public static final String RECEIPT_ID = "receipt-id";
 
 
+	@SuppressWarnings("serial")
 	private final Map<String, List<String>> headers;
 
 

spring-messaging/src/main/java/org/springframework/messaging/support/ErrorMessage.java

@@ -46,6 +46,7 @@ public class ErrorMessage extends GenericMessage<Throwable> {
 	private static final long serialVersionUID = -5470210965279837728L;
 
 	@Nullable
+	@SuppressWarnings("serial")
 	private final Message<?> originalMessage;
 
 

spring-messaging/src/main/java/org/springframework/messaging/support/GenericMessage.java

@@ -38,7 +38,7 @@ public class GenericMessage<T> implements Message<T>, Serializable {
 
 	private static final long serialVersionUID = 4268801052358035098L;
 
-
+	@SuppressWarnings("serial")
 	private final T payload;
 
 	private final MessageHeaders headers;

spring-messaging/src/test/java/org/springframework/messaging/protobuf/Msg.java

@@ -22,6 +22,7 @@ package org.springframework.messaging.protobuf;
 /**
  * Protobuf type {@code Msg}
  */
+@SuppressWarnings("serial")
 public  final class Msg extends
     com.google.protobuf.GeneratedMessage
     implements MsgOrBuilder {

spring-messaging/src/test/java/org/springframework/messaging/protobuf/SecondMsg.java

@@ -6,6 +6,7 @@ package org.springframework.messaging.protobuf;
 /**
  * Protobuf type {@code SecondMsg}
  */
+@SuppressWarnings("serial")
 public  final class SecondMsg extends
     com.google.protobuf.GeneratedMessage
     implements SecondMsgOrBuilder {