_access key 'true' in lowercase letters not granting access

Problem/Motivation

I used the drupal module upgrade for a simple module with a single route. Original Drupal 7 'access calback' is => TRUE:

array(
        'page callback' => 'zip_search_search',
        'type' => MENU_CALLBACK,
        'access callback' => TRUE
    );

The resulting requirements key in routes.yml is like this:

requirements:
    _access: 'true'

In my drupal 9 Project, this is not recognized as supposed. It needs to be:

requirements:
    _access: 'TRUE'

See https://www.drupal.org/docs/drupal-apis/routing-system/structure-of-routes

Steps to reproduce

dmu-upgrade a simple module with a page callback and 'access callback' => TRUE in its hook_menu.

Proposed resolution

Should output

requirements:
    _access: 'TRUE'