Add migration guide for MateriallScrollBehavior's new behavior

[camsim99]

Warning

Must land after flutter/flutter#178308.

Description of what this PR is changing or adding, and why:
Adds a migration guide to help people handle MaterialScrollBehavior adding a Scrollbar automatically on Android.

Issues fixed by this PR (if any):
This is part of flutter/flutter#170540.

PRs or commits this PR depends on (if any):
This migration guide is directly tied to changes that will be made when flutter/flutter#178308 lands.

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 6f6ac31):

https://flutter-docs-prod--pr12719-scrollbar-migration-xdk5q4x2.web.app

[sfshaza2]

Overall, this looks great. But @camsim99, can you add it to the index file? thx!

[camsim99]

Let's wait to land this so that @Piinks gets a chance to review and the PR this depends on is landed -- flutter/flutter#178308.

[Piinks]