build(deps): 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/draft-pdf.yaml

@@ -19,7 +19,7 @@ jobs:
           journal: joss
           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/test-coverage.yaml

@@ -56,7 +56,7 @@ jobs:
 
       - name: Upload test results
         if: failure()
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: coverage-test-failures
           path: ${{ runner.temp }}/package