Fix ArcanistFormattedStringXHPASTLinterRule on older PHP after D21500
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
Differential Revision: https://secure.phabricator.com/D21505