R1:2f1acf8b10cb

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…