Merged PR 47424: Refactor avoidable usages of protected access modifier

## What's being changed

We've replaced a `protected` access property with `private`.

## Why it's being changed

This was picked up by Adobe for their Assurance Program tests.

## How to review / test this change

- Enable your account, and enable chat
- Go to Marketing > Dotdigital Chat
- See an iframe appear at /admin/dotdigitalgroup_chat/studio/

Related work items: #221643

Controller/Adminhtml/Studio/Index.php

@@ -20,7 +20,7 @@ class Index extends Action
  /**
  * @var PageFactory
  */
- protected $resultPageFactory;
+ private $resultPageFactory;
 
  /**
  * @var Data