[docs] stop specifying a JDK version

[pq]

Stale and not adding value. (Destined to get stale again too.)

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[gemini-code-assist]

Code Review

This pull request removes the explicit mention of JDK version 23 from the environment setup section in CONTRIBUTING.md to reduce documentation staleness. Feedback indicates that several other hardcoded references to version 23 remain in the file, which are inconsistent with the project's actual requirement of JDK 21 and should be updated to generic placeholders.