[DX][UX] Convert the list of items in `update_info_page()` to a proper, translatable output, themed via `theme_item_list()`

[klonos]

I'm referring to this: https://github.com/backdrop/backdrop/blob/1.x/core/update.php#L317

  $output = '<p>Use this utility to update your site whenever you install a new version of Backdrop CMS or one of the site\'s modules.</p>';
  $output .= '<p>For more detailed information, see the <a href="https://backdropcms.org/upgrade">Upgrading Backdrop CMS</a> page. If you are unsure of what these terms mean, contact your hosting provider.</p>';
  $output .= '<p>Before running updates, the following steps are recommended.</p>';
  $output .= "<ol>\n";
  $output .= "<li><strong>Create backups.</strong> This update utility will alter your database and config files. In case of an emergency you may need to revert to a recent backup; make sure you have one.\n";
  $output .= "<ul>\n";
  $output .= "<li><strong>Database:</strong> Create a database dump of the '" . $db_name . "' database.</li>\n";
  $output .= "<li><strong>Config files:</strong> Back up the entire directory at '" . $config_dir . "'.</li>\n";
  $output .= "</ul>\n";
  $output .= '<li>Put your site into <a href="' . base_path() . '?q=admin/config/development/maintenance">maintenance mode</a>.</li>' . "\n";
  $output .= "<li>Install your new files into the appropriate location, as described in <a href=\"https://backdropcms.org/upgrade\">the handbook</a>.</li>\n";
  $output .= "</ol>\n";
  $output .= "<p>After performing the above steps proceed using the continue button.</p>\n";

This should be an array of proper list items, output via theme_item_list() instead.

[klonos]

...I'll file a PR for this, but it will be blocked on #5780 in order to allow nested list items to have different type of list from their parent.

[klonos]

PR here: https://github.com/backdrop/backdrop/pull/4822

Until #5780 is fixed, this should NOT be merged, because the nested list items inherit the parent type (<ol>) instead of what is specified in code (<ul>):