docs: add archive notice to README

[adityamaru]

This repository is being archived. Adding notice to README before archiving.

---

Note

Adds an archive notice to the README advising migration to the upstream-maintained action due to Blacksmith transparent caching.

• Docs:
  • Add an Archive Notice to README.md.
    • Notes Blacksmith transparent caching support and recommends switching to upstream-maintained action.
    • States this action will no longer receive dependency or security updates.

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