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

This PR reopens #8470, which was previously marked as stale and auto-closed.

Ubuntu 20.04 is EOL, and the latest supported EKS version for Ubuntu 20.04 was 1.29, which has also reached the end of its standard support.

Users should migrate to Ubuntu Pro 20.04 to continue to use extended support of 1.29.

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]

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