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-core/src/main/java/org/springframework/util/MimeType.java

@@ -102,6 +102,7 @@ public class MimeType implements Comparable<MimeType>, Serializable {
 
 	private final String subtype;
 
+	@SuppressWarnings("serial")
 	private final Map<String, String> parameters;
 
 	@Nullable

spring-core/src/main/java/org/springframework/util/UnmodifiableMultiValueMap.java

@@ -47,7 +47,7 @@ final class UnmodifiableMultiValueMap<K,V> implements MultiValueMap<K,V>, Serial
 
 	private static final long serialVersionUID = -8697084563854098920L;
 
-
+	@SuppressWarnings("serial")
 	private final MultiValueMap<K, V> delegate;
 
 	@Nullable
@@ -266,7 +266,7 @@ final class UnmodifiableMultiValueMap<K,V> implements MultiValueMap<K,V>, Serial
 
 		private static final long serialVersionUID = 2407578793783925203L;
 
-
+		@SuppressWarnings("serial")
 		private final Set<Entry<K, List<V>>> delegate;
 
 
@@ -531,6 +531,7 @@ final class UnmodifiableMultiValueMap<K,V> implements MultiValueMap<K,V>, Serial
 
 		private static final long serialVersionUID = 5518377583904339588L;
 
+		@SuppressWarnings("serial")
 		private final Collection<List<V>> delegate;
 
 

spring-core/src/test/java/org/springframework/core/serializer/SerializationConverterTests.java

@@ -71,7 +71,7 @@ class SerializationConverterTests {
 
 		private static final long serialVersionUID = 1L;
 
-		@SuppressWarnings("unused")
+		@SuppressWarnings({"unused", "serial"})
 		private Object object;
 	}