MDL-48618 grades: Do not run minmax upgrade for fresh installs

lib/db/install.php

@@ -130,6 +130,7 @@ function xmldb_main_install() {
         'stringfilters'         => '', // These two are managed in a strange way by the filters
         'filterall'             => 0, // setting page, so have to be initialised here.
         'texteditors'           => 'atto,tinymce,textarea',
+        'upgrade_minmaxgradestepignored' => 1, // New installs should not run this upgrade step.
     );
     foreach($defaults as $key => $value) {
         set_config($key, $value);