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-web/src/test/java/org/springframework/protobuf/Msg.java

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

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

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