fix: update GHA runner image to ubuntu-latest

[tekenstam]

This PR updates the runner image for all relevant GitHub Actions workflows to use ubuntu-latest instead of the deprecated ubuntu-20.04.

See actions/runner-images#11101 for more details.

Changes

• .github/workflows/unit-test.yml: runs-on updated from ubuntu-20.04 to ubuntu-latest
• .github/workflows/functional-test.yaml: runs-on updated from ubuntu-20.04 to ubuntu-latest (if previously present; now confirmed as ubuntu-latest)

Why?

• GitHub Actions is deprecating the ubuntu-20.04 runner image
• Ensures continued support and security updates for CI workflows
• Follows best practices for open-source and Kubernetes-related projects

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.13%. Comparing base (2945307) to head (128c17e).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #484   +/-   ##
=======================================
  Coverage   46.13%   46.13%           
=======================================
  Files          40       40           
  Lines        7004     7004           
=======================================
  Hits         3231     3231           
  Misses       3617     3617           
  Partials      156      156           

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