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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yml

@@ -218,7 +218,7 @@ jobs:
           ruby-version: "3.4"
           bundler-cache: true
       - run: ./bin/test-gem-build gems ruby
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         with:
           name: source-gem
           path: gems
@@ -281,7 +281,7 @@ jobs:
           docker run --rm -v $PWD:/work -w /work \
             ghcr.io/rake-compiler/rake-compiler-dock-image:${{ needs.native_setup.outputs.rcd_image_version }}-mri-${{ matrix.platform }} \
             ./bin/test-gem-build gems ${{ matrix.platform }}
-      - uses: actions/upload-artifact@v5
+      - uses: actions/upload-artifact@v6
         with:
           name: "cruby-${{ matrix.platform }}-gem"
           path: gems