Document JDK17 by default

[roaksoax]

What does this PR do?

Updates the docs to correctly display JDK17 as the default JDK version.

Why is it important/What is the impact to the user?

Current docs for 8.4 show that JDK11 is the default, however JDK17 was made default for 8.4

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

[github-actions[bot]]

:page_with_curl: DOCS PREVIEW :sparkles: https://logstash_14511.docs-preview.app.elstc.co/diff

[github-actions[bot]]

:page_with_curl: DOCS PREVIEW :sparkles: https://logstash_14511.docs-preview.app.elstc.co/diff