Add support for Ubuntu Pro 20.04 based EKS images

[toabctl]

Ubuntu Pro for 20.04 (Focal) is available for EKS 1.27 - 1.29 so add support for that to eksctl.

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)

[toabctl]

• I did a manual test (after building with make build) with: ./eksctl --verbose 5 create cluster --name toabctl-test-focalpro-1-29-try3 --version 1.29 --node-ami-family UbuntuPro2004 --ssh-access. that was successful.
• the current failures in the linter seem to be unrelated to my change.

[cheeseandcereal]

Thanks for the contribution! These changes look great to me. Thanks for adding the tests.