Fix ArcanistFormattedStringXHPASTLinterRule on older PHP after D21500

Summary:
Calling 'Foo::bar' is only supported since PHP 7, whereas the array form
is supported since PHP 5.4, which is below our PHP 5.5 baseline.

Test Plan: No regressions under PHP 8 and snippet tested on 3v4l

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21505