CI: Use .ruby-version Ruby in publish.yml

.github/workflows/publish.yml

@@ -12,10 +12,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Ruby 2.7
+      - name: Set up latest Ruby
         uses: ruby/setup-ruby@v1
-        with:
-          ruby-version: 2.7
 
       - name: Publish to RubyGems
         run: |