Add notice on v1 embedding removal to 3.29.0 breaking changes

[gmackall]

Description of what this PR is changing or adding, and why:

The deletion of the v1 embedding (flutter-team-archive/engine#51229) should have had an associated breaking changes notice. Unfortunately this was missed, under the assumption that a flutter tool check would have caught references to the v1 embedding in an earlier release, and provided upgrade instructions.

Issues fixed by this PR (if any):

flutter/flutter#164024

PRs or commits this PR depends on (if any):

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.

[reidbaker]

Consider making this a table instead of text.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 1d82ef2):

https://flutter-docs-prod--pr11749-v1-embedding-27nmggcc.web.app

[parlough]

Thanks for adding this notice/guide! It'll definitely be helpful :)

I made a few minor formatting and copy changes, but looks good to me!