::class is available since PHP 5.5 (5.5 is a minimal requirement by Phorge): https://www.php.net/manual/en/language.oop5.basic.php#language.oop5.basic.class.class
It makes finding code using IDEs easier; see discussion in D25500.
Thus replace all string return values with returning the ::class constant instead, with one exception: 'PhabricatorSettingApplication' in PhabricatorUserPreferencesSearchEngine.php does not exist and makes arc lint fail so this string remained unchanged.
Also note that two occurrences were wrapped in pht() for reasons I do not know.
List of functions whose return value get updated in this code change:
- getApplicationClassName()
- getAdapterApplicationClass()
- getDatasourceApplicationClass()
- getEditorApplicationClass()
- getEngineApplicationClass()
- getPHIDTypeApplicationClass()
- getQueryApplicationClass()
cf. T15158
