Bump actions/upload-artifact from 6 to 7

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 6 to 7.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v6...v7)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '7'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/build-paper.yaml

@@ -18,7 +18,7 @@ jobs:
           journal: joss
           paper-path: joss/paper.md
       - name: Upload
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v7
         with:
           name: paper
           path: joss/paper.pdf

.github/workflows/test-coverage.yaml

@@ -68,7 +68,7 @@ jobs:
 
       - name: Upload test results
         if: failure()
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v7
         with:
           name: coverage-test-failures
           path: ${{ runner.temp }}/package