Update changelog for AAD scope

[aavasthy]

Description

Updating changelog for AAD scope PR #42228

All SDK Contribution checklist:

• [X ] CHANGELOG is updated for new features, bug fixes or other significant changes.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• [X ] There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

[Copilot]

Pull Request Overview

This PR updates the changelog to document a new feature that allows overriding AAD audience scope through an environment variable. The change is part of documenting PR #42228 which added this capability.

• Added changelog entry for AAD scope override functionality