Change key of custom block vertical tabs to use 'advanced' instead of 'additional_settings'

Follow up for #1871772-108: Convert custom blocks to content entities

Problem/Motivation

+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
@@ -0,0 +1,216 @@
+ $form['additional_settings'] = array(
+ '#type' => 'vertical_tabs',
Our new standard is to call the vertical tabs container 'advanced' instead of additional_settings. Let's adjust that in a follow-up.

Proposed resolution

Rename from 'additional_settings' to 'advanced'

Remaining tasks

Write a patch