CI: 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/macvim-buildtest.yaml

@@ -413,7 +413,7 @@ jobs:
       # and add pictures to make them look nice. 
       - name: Upload MacVim image
         if: inputs.publish && (startsWith(github.ref, 'refs/tags/') || github.ref == 'refs/heads/master')
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: MacVim${{ inputs.publish_postfix  }}.dmg
           path: src/MacVim/build/Release/MacVim${{ inputs.publish_postfix }}.dmg