chore: bump unity-test-runner version in v3 docs

[davidschreiber]

Changes

• Bumps all usages of the game-ci/unity-test-runner GitHub action to the newer v3.

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [x] Readme (updated or not needed)
• [x] Tests (added, updated or not needed)

[github-actions[bot]]

[webbertakken]

Thank you!