Moodle 4.3 final deprecation of *_get_completion_state() callbacks

[marcestone]

Hi!
I found that in https://github.com/PoetOS/moodle-mod_questionnaire/blob/MOODLE_401_STABLE/lib.php#L1230, the questionnaire_get_completion_state function is still declared in the latest version. According to the Moodle Tracker ticket https://tracker.moodle.org/browse/MDL-71196, the *_get_completion_state() callbacks were deprecated and will be replaced by the activity custom completion classes.
Here it is how the callbacks were replaced in multiple Moodle Core plugins: moodle/moodle@334b6c7.