Install RuboCop only on Ruby 2.6+

Run RuboCop on Ruby 2.6

Author: mishina2228

.github/workflows/rubocop.yml

@@ -9,10 +9,10 @@ jobs:
       BUNDLE_WITHOUT: development
     steps:
     - uses: actions/checkout@v3
-    - name: Set up Ruby 2.4
+    - name: Set up Ruby 2.6
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: 2.4
+        ruby-version: 2.6
         bundler-cache: true # runs 'bundle install' and caches installed gems automatically
     - name: Run RuboCop
       run: bundle exec rubocop

Gemfile

@@ -19,7 +19,7 @@ group :test do
   # Downgrade psych because old RuboCop can't use new Psych
   gem 'psych', '< 4.0.0'
   # https://github.com/bbatsov/rubocop/pull/4789
-  gem 'rubocop', '~> 1.30'
+  gem 'rubocop', '~> 1.30' if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.6')
 end
 
 group :benchmarks, optional: true do