8229997: Apply java.io.Serial annotations in java.base

Reviewed-by: alanb, rriggs

src/java.base/share/classes/java/util/HashSet.java

@@ -91,6 +91,7 @@ public class HashSet<E>
     extends AbstractSet<E>
     implements Set<E>, Cloneable, java.io.Serializable
 {
+    @java.io.Serial
     static final long serialVersionUID = -5024744406713321676L;
 
     private transient HashMap<E,Object> map;
@@ -271,6 +272,7 @@ public class HashSet<E>
      *             (int), followed by all of its elements (each an Object) in
      *             no particular order.
      */
+    @java.io.Serial
     private void writeObject(java.io.ObjectOutputStream s)
         throws java.io.IOException {
         // Write out any hidden serialization magic
@@ -292,6 +294,7 @@ public class HashSet<E>
      * Reconstitute the {@code HashSet} instance from a stream (that is,
      * deserialize it).
      */
+    @java.io.Serial
     private void readObject(java.io.ObjectInputStream s)
         throws java.io.IOException, ClassNotFoundException {
         // Read in any hidden serialization magic