[JENKINS-50124] restore AbstractTaskListeners serialVersionUID.

If a subclass was serialized (default java serialization) then the
changes do push down the method to the interface would break
deserialisation when in reality the classes are still compatable.

Restoring the compatability by hard coding the serialVersionUID to what
it was before the change.

core/src/main/java/hudson/util/AbstractTaskListener.java

@@ -13,4 +13,7 @@ import org.kohsuke.accmod.restrictions.NoExternalUse;
 @Restricted(NoExternalUse.class)
 @RestrictedSince("2.91")
 public abstract class AbstractTaskListener implements TaskListener {
+
+    private static final long serialVersionUID = 7217626701881006422L;
+
 }