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/main/java/org/springframework/http/HttpHeaders.java

@@ -420,6 +420,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 	};
 
 
+	@SuppressWarnings("serial")
 	final MultiValueMap<String, String> headers;
 
 

spring-web/src/main/java/org/springframework/http/ReadOnlyHttpHeaders.java

@@ -43,6 +43,7 @@ class ReadOnlyHttpHeaders extends HttpHeaders {
 	private MediaType cachedContentType;
 
 	@Nullable
+	@SuppressWarnings("serial")
 	private List<MediaType> cachedAccept;
 
 

spring-web/src/main/java/org/springframework/web/client/RestClientResponseException.java

@@ -54,6 +54,7 @@ public class RestClientResponseException extends RestClientException {
 	private final String responseCharset;
 
 	@Nullable
+	@SuppressWarnings("serial")
 	private Function<ResolvableType, ?> bodyConvertFunction;
 
 

spring-web/src/main/java/org/springframework/web/client/UnknownContentTypeException.java

@@ -36,7 +36,7 @@ public class UnknownContentTypeException extends RestClientException {
 
 	private static final long serialVersionUID = 2759516676367274084L;
 
-
+	@SuppressWarnings("serial")
 	private final Type targetType;
 
 	private final MediaType contentType;