CI: harmonize exact version of Ruby with Gemfile

...until we are up to date with latest 3.3.6.

.github/workflows/main.yml

@@ -27,7 +27,7 @@ jobs:
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 3.3
+          ruby-version: 3.3.5
           bundler-cache: true # 'bundle install' and cache gems
       - name: Compile
         run: |