Disable Inline Form Errors
The core issue you linked to has a patch that is apparently working, so it may be easier to patch with Composer instead of adding code to a custom module.
But, if you want to use the $form['#disable_inline_form_errors'] = TRUE;, this is what you can do:
First, create a custom module.  You need to follow the instructions on the first four pages:

Getting Started - Background &amp; Prerequisites
Prepare a Module skeleton
Naming and placing your Drupal module
Let Drupal know about your module with an .info.yml file

Then, in the MYMODULE.module file, add this code:
/**
 * Customize the user login form.
 */
function MYMODULE_form_user_login_form_alter(&amp;$form, FormStateInterface $form_state, $form_id) {
  $form['#disable_inline_form_errors'] = TRUE;
}

What this code does is use hook_form_alter to target the user login form.  Lots of Drupal's form behaviors can be customized in this way.
You then need to go to the module administration page and enable your custom module.