[JENKINS-61046] - Allow plugins to force an update of the UpdateCenter (#4488)

* Allow plugins to force an update of the UpdateCenter

* Add javadoc to updateDirectlyNow

* [JENKINS-61046] introduce updateDirectlyNow() and updateDirectly()

@oleg-nenashev notes that the public API should not expose the security
option that is for testing.  Introduced updateDirectlyNow() as the new
API and updateDirectly() to have a mirror and deprecated
updateDirectly(boolean) as a consequence.

* fix javadoc grammar

fixes the grammar of the UpdateSite#updateDirectlyNow javadoc

Co-Authored-By: Raihaan Shouhell <raihaanhimself@gmail.com>

Co-authored-by: Raihaan Shouhell <raihaanhimself@gmail.com>

core/src/main/java/hudson/PluginManager.java

@@ -1762,7 +1762,7 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
 
     private FormValidation checkUpdatesServer() throws Exception {
         for (UpdateSite site : Jenkins.get().getUpdateCenter().getSites()) {
-            FormValidation v = site.updateDirectlyNow(DownloadService.signatureCheck);
+            FormValidation v = site.updateDirectlyNow();
             if (v.kind != FormValidation.Kind.OK) {
                 // Stop with an error
                 return v;