Bump actions/upload-artifact from 5 to 6 (#33)

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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/cross-compile.yml

@@ -40,7 +40,7 @@ jobs:
           platform: ${{ matrix.platform }}
           ruby-versions: "2.7,3.0,3.1,3.2,3.3,3.4"
 
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         with:
           name: mrml-ruby-${{ matrix.platform }}
           path: ${{ steps.cross-gem.outputs.gem-path }}
@@ -49,7 +49,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: cross-gem
     steps:
-      - uses: actions/upload-artifact/merge@v5
+      - uses: actions/upload-artifact/merge@v6
         with:
           name: cross-gem
           pattern: mrml-ruby-*