ci.yml workflow: bump upload-artifact to v4

[soxofaan]

v3 of actions/upload-artifact or actions/download-artifact are going away end of Januari: https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/

I was grepping through my repo's and noticed my local dirty-equals clone was still on v3: https://github.com/samuelcolvin/dirty-equals/blob/065162ac195964df7f5c47288561809e49deac48/.github/workflows/ci.yml#L94

[samuelcolvin]

sorry for the slow response, you need to change actions/download-artifact too.

[lucascolley]

sorry for the slow response, you need to change actions/download-artifact too.

this is now done in gh-116