MDL-45242 Lib: Replace calls to deprecated functions

In all cases changes have been kept to a minimum while not making
the code completely horrible. For example, there are many instances
where it would probably be better to rewrite a query entirely, but
I have not done that (in order to reduce the risk of changes).

lib/classes/check/access/riskadmin.php

@@ -66,7 +66,8 @@ class riskadmin extends check {
      */
     public function get_result(): result {
         global $DB, $CFG;
-        $userfields = \user_picture::fields('u');
+        $userfieldsapi = \core\user_fields::for_userpic();
+        $userfields = $userfieldsapi->get_sql('u', false, '', '', false)->selects;
         $sql = "SELECT $userfields
                   FROM {user} u
                  WHERE u.id IN ($CFG->siteadmins)";