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]

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