Drop support for Ubuntu 20.04 images

[JessicaJang]

Ubuntu 20.04 (Focal Fossa) reached end-of-life
on May 31, 2025.

Discontinue support for this EOL release.

Description

Remove Ubuntu2004 from the files.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[github-actions]

Hello JessicaJang 👋 Thank you for opening a Pull Request in eksctl project. The team will review the Pull Request and aim to respond within 1-10 business days. Meanwhile, please read about the Contribution and Code of Conduct guidelines here. You can find out more information about eksctl on our website

[JessicaJang]

could someone review this?

[cheeseandcereal]

Hey, sorry for the delay, and thanks for the PR! The changes largely look good, but there are some conflicts. Would you mind rebasing? And we can get back and merge this. Thanks!