Bump actions/upload-artifact to v4

Author: ashmaroli

.github/workflows/gem_artifact.yml

@@ -31,7 +31,7 @@ jobs:
       - name: Build Gem
         run: gem build jekyll-admin.gemspec
       - name: Stash Gem as Temporary Artifact
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v4
         with:
           name: "jekyll-admin.gem"
           path: "${{ github.workspace }}/*.gem"