Bump actions/upload-artifact from 5 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/ci-pr.yaml

@@ -74,7 +74,7 @@ jobs:
           mike set-default $MIKE_VERSION
 
       - name: Upload site
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: docs-site
           path: site/

.github/workflows/ci.yaml

@@ -119,7 +119,7 @@ jobs:
         run: python -m build
 
       - name: Upload distribution files
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: dist-packages
           path: dist/
@@ -221,7 +221,7 @@ jobs:
           mike set-default $MIKE_VERSION
 
       - name: Upload site
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: docs-site
           path: site/