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/render_readme.yml

@@ -89,7 +89,7 @@ jobs:
         shell: Rscript {0}
 
       - name: Upload README.md as an artifact
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: readme
           path: README.md

.github/workflows/test-coverage.yaml

@@ -43,7 +43,7 @@ jobs:
         shell: Rscript {0}
 
       - name: Upload coverage data
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: coverage-data
           path: coverage.xml