jobqueue: Deprecate JobQueueGroup::waitForBackups()

No known usage found per CodeSearch and we should now instead use
the equivalent in JobQueue.

Change-Id: I7d8fd767153439333080843bcfe40a2267e72c12

RELEASE-NOTES-1.41

@@ -499,6 +499,8 @@ because of Phabricator reports.
   $wgInterwikiCache setting instead.
 * SiteConfiguration::extractVar() and ::extractGlobal() have been deprecated
   and also emit deprecation warnings.
+* JobQueueGroup::waitForBackups() is deprecated. JobQueue::waitForBackups()
+  should be used instead.
 * Hooks::isRegistered(), ::getHandlers(), ::run() and ::runWithoutAbort(),
   deprecated in 1.35, now emit deprecation warnings.
 * VirtualRESTService class and its subclasses now emit deprecation warnings

includes/jobqueue/JobQueueGroup.php

@@ -284,9 +284,12 @@ class JobQueueGroup {
 	 *
 	 * This does nothing for certain queue classes.
 	 *
+	 * @deprecated since 1.41, use JobQueue::waitForBackups() instead.
+	 *
 	 * @return void
 	 */
 	public function waitForBackups() {
+		wfDeprecated( __METHOD__, '1.41' );
 		// Try to avoid doing this more than once per queue storage medium
 		foreach ( $this->jobTypeConfiguration as $type => $conf ) {
 			$this->get( $type )->waitForBackups();