Adds migration guide for default cache extent change

[chunhtai]

for flutter/flutter#182905

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 b3d0916):

https://flutter-docs-prod--pr13177-cache-default-th9gvcrr.web.app

[sfshaza2]

I know that this is still in draft form, but lgtm, so it's ready when you are.