Reimagine setup-java README (#1192)

* Implement new feature for user authentication and improve error handling

* Clarify README review feedback

Clarify distribution case sensitivity, GHES token defaults, and cache key placeholder notation in the README.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: f0ff1923-c6ca-472e-83b2-3a813ec6d781

* Update README to clarify V6 development status and enhance contributions section

* Update README to recommend permissions for setup-java action in GitHub Actions

* Restore README usage heading

Rename the quick-start section to the conventional Usage heading used by setup actions.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: f0ff1923-c6ca-472e-83b2-3a813ec6d781

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Copilot-Session: f0ff1923-c6ca-472e-83b2-3a813ec6d781

Author: brunoborges