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.yml

@@ -55,7 +55,7 @@ jobs:
       #   with:
       #     files: ./cov-html/libopenlibm.info
       #     token: ${{ secrets.CODECOV_TOKEN }}
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         with:
           name: code-coverage-report-old
           path: ./cov-html/
@@ -87,7 +87,7 @@ jobs:
         with:
           files: ./cov-html/libopenlibm.info
           token: ${{ secrets.CODECOV_TOKEN }}
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         with:
           name: code-coverage-report
           path: ./cov-html/