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-test/src/main/java/org/springframework/test/context/MergedContextConfiguration.java

@@ -84,6 +84,7 @@ public class MergedContextConfiguration implements Serializable {
 
 	private final Class<?>[] classes;
 
+	@SuppressWarnings("serial")
 	private final Set<Class<? extends ApplicationContextInitializer<?>>> contextInitializerClasses;
 
 	private final String[] activeProfiles;
@@ -92,11 +93,14 @@ public class MergedContextConfiguration implements Serializable {
 
 	private final String[] propertySourceProperties;
 
+	@SuppressWarnings("serial")
 	private final Set<ContextCustomizer> contextCustomizers;
 
+	@SuppressWarnings("serial")
 	private final ContextLoader contextLoader;
 
 	@Nullable
+	@SuppressWarnings("serial")
 	private final CacheAwareContextLoaderDelegate cacheAwareContextLoaderDelegate;
 
 	@Nullable

spring-test/src/main/java/org/springframework/test/context/support/DefaultTestContext.java

@@ -40,6 +40,7 @@ import org.springframework.util.StringUtils;
  * @author Rob Harrop
  * @since 4.0
  */
+@SuppressWarnings("serial")
 public class DefaultTestContext implements TestContext {
 
 	private static final long serialVersionUID = -5827157174866681233L;