chore: Bump linting action Ubuntu version

[dougch]

Problem:

Github is deprecating the Ubuntu20 runner on 2025-04-01. We've pinned to this version for cppcheck

Reason:
"We maintain the latest two stable versions of any given OS version. Ubuntu 24.04 is going GA on 2025-02-01 so we start deprecating the oldest image at that time."

Solution:

Bump the version to ubuntu-latest, which might require bumping the cppcheck version as well.