Fix check in PhabricatorMySQLSetupCheck:shouldUseMySQLSearchEngine()
Summary:
Per PHPStan static code analysis, currently
`Instanceof between PhabricatorSearchService and PhabricatorMySQLSearchHost will always evaluate to false`.
Per discussion in the task and per testing, make the check work.
Closes T15891
Test Plan: Probably set up a Phorge instance with recent MariaDB/MySQL, and set up a Phorge instance with ancient DB versions (MySQL 5.6.3 or older; MariaDB 10.0 or older) not supporting FULLTEXT indexes for InnoDB tables?
Reviewers: O1 Blessed Committers, 20after4
Reviewed By: O1 Blessed Committers, 20after4
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15891
Differential Revision: https://we.phorge.it/D25975