Soft deprecate PageProps::getInstance()

PageProps has been converted into a service with dependency injection.
PageProps::getInstance() relies on global state and thus should be
avoided.

Bug: T289544
Change-Id: Iab3e94059cb6d2d11950ae870a1713f4b3a738fb

RELEASE-NOTES-1.38

@@ -125,6 +125,8 @@ because of Phabricator reports.
   GrantsLocalization services.
 * The global functions wfReadOnly() and wfReadOnlyReason() have been
   deprecated in favor of the ReadOnlyMode service.
+* PageProps::getInstance() has been deprecated. Use
+  MediaWikiServices::getPageProps() instead.
 * Skin::getSkinStylePath has been hard deprecated. Direct string path
   should be used instead.
 * SkinTemplate::getPersonalToolsList(), deprecated since 1.35, now emits

includes/PageProps.php

@@ -46,6 +46,8 @@ class PageProps {
 	private $cache;
 
 	/**
+	 * @deprecated since 1.38, use MediaWikiServices::getPageProps() instead
+	 *
 	 * @return PageProps
 	 */
 	public static function getInstance() {