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] <[email protected]>

Author: dependabot[bot]

.github/workflows/build_joss_paper.yml

@@ -16,7 +16,7 @@ jobs:
           paper-path: ./joss-paper/paper.md
 
       - name: Upload
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: paper
           path: ./joss-paper/paper.pdf

.github/workflows/main.yml

@@ -47,7 +47,7 @@ jobs:
 
     - name: Upload HTML report.
       if: matrix.python-version == '3.10' && matrix.os == 'ubuntu-latest'
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       with:
         name: html-report
         path: htmlcov

.github/workflows/pypi.yml

@@ -11,7 +11,7 @@ jobs:
     - name: Build SDist and wheel
       run: pipx run build
 
-    - uses: actions/upload-artifact@v5
+    - uses: actions/upload-artifact@v6
       with:
         path: dist/*