Suppress PHP 8 deprecation warning in startup

Summary: Ref T13588. See D21497. As of PHP 8, the XML entity loader is disabled by default and the `libxml_disable_entity_loader` function is deprecated. Thus suppress the deprecation warning for now; we could skip the function call, but this is safer.

Test Plan:
* Still works with PHP 7.
* No more deprecation message with PHP 8.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21701

Author: 0

support/startup/PhabricatorStartup.php

@@ -392,8 +392,11 @@ private static function setupPHP() {
     ini_set('memory_limit', -1);
 
     // If we have libxml, disable the incredibly dangerous entity loader.
+    // PHP 8 deprecates this function and disables this by default; remove once
+    // PHP 7 is no longer supported or a future version has removed the function
+    // entirely.
     if (function_exists('libxml_disable_entity_loader')) {
-      libxml_disable_entity_loader(true);
+      @libxml_disable_entity_loader(true);
     }
 
     // See T13060. If the locale for this process (the parent process) is not