MDL-31330 admin: fix incorrect context use

This was introduced by MDL-31006, being backported to 21_STABLE
and that use escaped the (my) scrutiny :-( (the new access API is
22_STABLE and upwards only)

admin/enrol.php

@@ -31,7 +31,7 @@
 $confirm = optional_param('confirm', 0, PARAM_BOOL);
 
 $PAGE->set_url('/admin/enrol.php');
-$PAGE->set_context(context_system::instance());
+$PAGE->set_context(get_context_instance(CONTEXT_SYSTEM));
 
 require_login();
 require_capability('moodle/site:config', get_context_instance(CONTEXT_SYSTEM));
@@ -119,4 +119,4 @@
 }
 
 
-redirect($return);
+redirect($return);