feat: add ubuntu 26.04

[janishorsts]

The new Ubuntu LTS has been released: https://releases.ubuntu.com/resolute/.

This PR adds support for this LTS version.

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	25
This PR	25
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.

[gemini-code-assist]

Code Review

This pull request introduces support for Ubuntu 26.04, including documentation updates and a new Earthfile. It also standardizes comments across existing OS Earthfiles. The review feedback identifies a recurring grammatical error in the updated comments ('an dind' should be 'a dind') and recommends adding the generic common+test target to several existing Earthfiles to ensure consistent and comprehensive test coverage across all supported distributions.