Address PR feedback

- Remove @exported from deprecated properties lengthName & backupVersion
- Fix Issue link in PluginManagerTest from JENKINS-27993 to issues/21047

Author: himanshu748

core/src/main/java/hudson/PluginWrapper.java

@@ -663,7 +663,7 @@ public String toString() {
      *
      * @deprecated For most purposes, use {@link #getDisplayName()}.
      */
-    @Exported(visibility = 2)
+    @Exported
     @Deprecated
     public String getLongName() {
         String name = manifest.getMainAttributes().getValue("Long-Name");