Add migration guide for MateriallScrollBehavior's new behavior

[camsim99]

[!WARNING]
Must land after https://github.com/flutter/flutter/pull/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 https://github.com/flutter/flutter/issues/170540.

PRs or commits this PR depends on (if any): This migration guide is directly tied to changes that will be made when https://github.com/flutter/flutter/pull/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 6f6ac3135dc8c40f4cfc960d13d997ca33c195ee):

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

[camsim99]

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

[sfshaza2]

Don't land until the next stable release...