MDL-66968 php74: array_key_exists() for objects is deprecated

Replace it for correct property_exists() when the element
being inspected is a property of object/class.

Amended and squased changes:
- keep mongo unmodified. The information is array, hence correct.
- fix a couple of messaging phpdocs that were incorrect.

Amended take#2:
- As far as mongo resturns BSONDocument that is ArrayObject, aka
implements ArrayAccess, we have decided to explicitly cast results
to array so existing array_key_exists() and other accesses will
continue working the same.

competency/classes/api.php

@@ -3839,7 +3839,7 @@ class api {
         if (!$userevidence->can_manage()) {
             throw new required_capability_exception($context, 'moodle/competency:userevidencemanage', 'nopermissions', '');
 
-        } else if (array_key_exists('userid', $data) && $data->userid != $userevidence->get('userid')) {
+        } else if (property_exists($data, 'userid') && $data->userid != $userevidence->get('userid')) {
             throw new coding_exception('Can not change the userid of a user evidence.');
         }