Merge forwardport of #11254 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/11254.patch (created by @romainruaud) based on commit(s):
  1. 43cfc74

Fixed GitHub Issues in 2.3-develop branch:
  - #10317: Region is being overridden when changing from a required-state country to one that is not required (reported by @nei)

app/code/Magento/Checkout/view/frontend/web/js/region-updater.js

@@ -206,7 +206,7 @@ define([
                     regionInput.removeClass('required-entry');
                 }
 
-                regionList.removeClass('required-entry').hide();
+                regionList.removeClass('required-entry').prop('disabled', 'disabled').hide();
                 regionInput.show();
                 label.attr('for', regionInput.attr('id'));
             }