Run RuboCop CI step on Ruby 3.3

The gemset is becoming uninstallable on Ruby 2.7 and at least requires
Ruby 3.1. Version 3.3 is picked since it's the latest version supported
in the CI matrix.

Author: ekohl

.github/workflows/rubocop.yml

@@ -9,7 +9,7 @@ jobs:
         uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 2.7.0
+          ruby-version: 3.3
       - name: rubocop
         uses: reviewdog/action-rubocop@v2
         with: