[stable] GHA: Bump actions/{checkout,{up,down}load-artifact} versions

[kinke]

To get rid of deprecations and hopefully benefit from improved performance for up/download-artifact v4.

[dlang-bot]

Thanks for your pull request and interest in making D better, @kinke! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[kinke]

Oh wow, there's definitely a huge performance gain for the documentation-artifacts at least (1432 files, ~15 MB). Comparing 2 ~simultaneous pipelines, Linux jobs each:

• Upload: 3 secs now (https://github.com/dlang/installer/actions/runs/9025218401/job/24800545422?pr=577) vs. 53 secs (https://github.com/dlang/installer/actions/runs/9025225631/job/24800566991?pr=576)
• Download: 1 second now (https://github.com/dlang/installer/actions/runs/9025218401/job/24800635188?pr=577) vs. 92 secs (https://github.com/dlang/installer/actions/runs/9025225631/job/24800683220?pr=576)