The form element for the ID property must be disabled when the user restriction is not new

Problem/Motivation

The form element for the ID (name in the 2.0.x branch) property must be disabled when the user restriction is edited.

This is what done for other configuration entities, for example filter_format.

    $form['format'] = [
      '#type' => 'machine_name',
      '#required' => TRUE,
      '#default_value' => $format->id(),
      '#maxlength' => 255,
      '#machine_name' => [
        'exists' => [
          $this,
          'exists',
        ],
        'source' => [
          'name',
        ],
      ],
      '#disabled' => !$format->isNew(),
      '#weight' => -20,
    ];

Proposed resolution

Change the form element for the ID property to be disabled when the user restriction is edited.