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