Bump actions/upload-artifact from 4 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 4 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v4...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] <[email protected]>

Author: dependabot[bot]

.github/workflows/build.yml

@@ -37,7 +37,7 @@ jobs:
 
       - name: Upload the PULL REQUEST number
         if: ${{ github.event_name == 'pull_request' }}
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: pull_request_number
           path: ./pull_request_number.txt
@@ -60,7 +60,7 @@ jobs:
             make -C . html
 
       - name: Upload HTML
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
             name: DocumentationHTML
             path: _build/html/