GH Actions: 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/playground.yml

@@ -97,7 +97,7 @@ jobs:
       # Upload the FOLDER (not a .zip). The artifact service zips it for us,
       # keeping the top-level folder name inside the archive.
       - name: Upload plugin artifact
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@v6
         with:
           name: ${{ secrets.DOT_ORG_SLUG }}
           path: ${{ steps.prep.outputs.PKG_DIR }}