Change flutter.dev/go/expansible-state-mixin to flutter.dev/go/codeshare-expansion-tile

[victorsanni]

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• 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).
• This PR uses semantic line breaks of 80 characters or fewer.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit b4b72ca):

https://flutter-docs-prod--pr11741-patch-3-96ms83gj.web.app

[parlough]

Thanks!

I've gone ahead and kept the old redirect pointing to the new one, just in case anyone already has the link somewhere :)