Setup Checks: Add a comment about mbstring.func_overload removal
Summary:
Once Phorge requires PHP 8 the check for mbstring.func_overload can be removed:
* https://www.php.net/manual/en/mbstring.configuration.php#ini.mbstring.func-overload
* https://www.php.net/manual/en/mbstring.overload.php
Document that so future maintainers can find this when running grep.
Test Plan: None; it's a comment only.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26283