docs: upgrades to Java 21

[hartmut-co-uk]

❓ Type of change

• [x] 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [x] 👌 Enhancement (improving an existing functionality like performance)
• [ ] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description, Motivation and Context

🧪 How Has This Been Tested?

Build, tests, pipeline succeeds.

📝 Checklist

• [ ] I have linked an issue or discussion.
• [x] I have updated the documentation accordingly.