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-aop/src/test/java/org/springframework/aop/target/PrototypeBasedTargetSourceTests.java

@@ -71,7 +71,7 @@ public class PrototypeBasedTargetSourceTests {
 		 * Nonserializable test field to check that subclass
 		 * state can't prevent serialization from working
 		 */
-		@SuppressWarnings("unused")
+		@SuppressWarnings({"unused", "serial"})
 		private TestBean thisFieldIsNotSerializable = new TestBean();
 
 		@Override