Fix GitHub Actions JDK setup label

[codingkiddo]

Summary

Updates the GitHub Actions setup step label from Set up JDK 1.11 to Set up JDK 21.

Motivation

The workflow already uses java-version: '21', but the step name still says JDK 1.11. This can confuse contributors when setting up the project locally, especially because the test runtime uses newer dependencies.

Changes

• Renamed the GitHub Actions Java setup step to match the configured JDK version.
• No build, dependency, or Java compatibility changes.

Testing

Configuration-only change. Existing workflow still uses JDK 21.

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[ryber]

Thanks @codingkiddo 🎉