docs: Add archive notice to README

[rekmarks]

Adds the archive notice to the README for this repository.

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Note

Updates README to state the package is migrated and the repo is archived, directing future development to the core monorepo.

• Docs:
  • README: Replaces migration-in-progress notice with an archive/migration notice, clarifying the package now lives in https://github.com/MetaMask/core and future development occurs there.

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