chore: add phpunit cache to gitignore

[crnkovic]

Summary

I noticed when I run tests that .phpunit.cache file is there to be committed. Maybe they changed something as it was previously called .phpunit.result.cache

Checklist

• I checked my UI changes against the design and there are no notable differences
• I checked my UI changes for any responsiveness issues
• I checked my (code) changes for obvious issues, debug statements and commented code
• I opened a corresponding card on Clickup for any remaining TODOs in my code
• I added a short description on how to test this PR (if necessary)
• I added a storybook entry for the component that was added (if necessary)
• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged