Mark Java 21 default in docs.

[mashhurs]

Release notes

[rn:skip]

What does this PR do?

Updates the JVM doc saying Java 17 is default. Unfortunately we changed it to Java 21 a way back.

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

Deliver accurate message about JDK

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

Author's Checklist

• [ ]

How to test this PR locally

Related issues

Use cases

Screenshots

Logs

[github-actions]

Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[github-actions]

📃 DOCS PREVIEW ✨ https://logstash_bk_17046.docs-preview.app.elstc.co/diff

[yaauie]

The asciidoc for main are frozen now; can you re-open a new PR against 8.x?

[mashhurs]

The asciidoc for main are frozen now; can you re-open a new PR against 8.x?

8.x: #17047
9.0: #17048

[robbavey]

Related: #16608

[github-actions]

Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[github-actions]

📃 DOCS PREVIEW ✨ https://logstash_bk_17046.docs-preview.app.elstc.co/diff

[mergify]

This pull request does not have a backport label. Could you fix it @mashhurs? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[mergify]

This pull request is now in conflicts. Could you fix it @mashhurs? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b docs-make-jdk12-default upstream/docs-make-jdk12-default
git merge upstream/main
git push upstream docs-make-jdk12-default