Bump GHA runners to ubuntu-latest

[mstoykov]

What?

Use the same github ubuntu runner (ubuntu-latest) for running tests as well.

Why?

This was previously disabled due to browsers tests were failing. But it seems like this is no longer needed.

Checklist

• [ ] I have performed a self-review of my code.
• [ ] I have commented on my code, particularly in hard-to-understand areas.
• [ ] I have added tests for my changes.
• [ ] I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• [ ] I have added the correct milestone and labels to the PR.
• [ ] I have updated the release notes: link
• [ ] I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• [ ] I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

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