build(deps): bump actions/upload-pages-artifact from 2.0.0 to 3.0.0

Bumps [actions/upload-pages-artifact](https://github.com/actions/upload-pages-artifact) from 2.0.0 to 3.0.0.
- [Release notes](https://github.com/actions/upload-pages-artifact/releases)
- [Commits](actions/upload-pages-artifact@v2.0.0...v3.0.0)

---
updated-dependencies:
- dependency-name: actions/upload-pages-artifact
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/ci.yml

@@ -49,7 +49,7 @@ jobs:
           # Ignore failures for older Nim
           cp htmldocs/{the,}index.html || true
 
-      - uses: actions/upload-pages-artifact@v2.0.0
+      - uses: actions/upload-pages-artifact@v3.0.0
         with:
           path: union/htmldocs