CI: use ruby/setup-ruby in Publish Workflow

This is for consistency, we are not using any of the specific features.

.github/workflows/publish.yml

@@ -13,9 +13,9 @@ jobs:
       - uses: actions/checkout@master
 
       - name: Set up Ruby 2.7
-        uses: actions/setup-ruby@v1
+        uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 2.7.x
+          ruby-version: 2.7
 
       - name: Publish to RubyGems
         run: |