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, recommending migration to the upstream-maintained action leveraging Blacksmith transparent caching.

• Documentation:
  • README.md:
    • Add an Archive Notice section.
    • Recommend switching to upstream-maintained versions due to Blacksmith transparent caching.
    • Note that this action will no longer receive dependency or security updates.

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