Update README with archiving notice and new link

[julien-marchand]

Added a warning about repository archiving and redirection.

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Note

Low Risk
Low risk documentation-only change that adds an archive banner and redirects readers to the maintained repository.

Overview
Adds a top-of-README WARNING banner stating the repository is archived and no longer maintained, and directs readers to Consensys/linea-monorepo for the latest Linea specification implementation.

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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