Drop Ruby 2.4 support

RuboCop does not support Ruby 2.4.
rubocop/rubocop#9648

And toshimaru#111 requires this PR.
Therefore, this PR drops support for Ruby 2.4 to enable the same cops as rails/rails.

.github/workflows/test.yml

@@ -4,17 +4,14 @@ jobs:
   test:
     strategy:
       matrix:
-        ruby: [2.4, 2.5, 2.6, 2.7, 3.0]
+        ruby: [2.5, 2.6, 2.7, 3.0]
         gemfile:
           - gemfiles/rubocop_1.8.gemfile
           - gemfiles/rubocop_1.9.gemfile
           - gemfiles/rubocop_1.10.gemfile
           - gemfiles/rubocop_1.11.gemfile
           - gemfiles/rubocop_1.12.gemfile
           - gemfiles/rubocop_1.13.gemfile
-        exclude:
-          - ruby: 2.4
-            gemfile: gemfiles/rubocop_1.13.gemfile
     env:
       BUNDLE_GEMFILE: ${{ matrix.gemfile }}
     runs-on: ubuntu-latest

README.md

@@ -35,7 +35,7 @@ $ rails generate rubocop_rails_config:install
 
 ### TargetRubyVersion
 
-Although Rails 7 (edge) only supports Ruby 2.7 or more, rubocop-rails_config still supports Ruby 2.4 or more to support as many Ruby versions as possible.
+Although Rails 7 (edge) only supports Ruby 2.7 or more, rubocop-rails_config still supports Ruby 2.5 or more to support as many Ruby versions as possible.
 
 If you'd like to change `TargetRubyVersion`, see [Customization](#customization).
 

rubocop-rails_config.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |spec|
   spec.files                 = Dir["README.md", "LICENSE", "config/*.yml", "lib/**/*"]
   spec.homepage              = "https://github.com/toshimaru/rubocop-rails_config"
   spec.license               = "MIT"
-  spec.required_ruby_version = ">= 2.4.0"
+  spec.required_ruby_version = ">= 2.5.0"
 
   spec.add_dependency "rubocop", ">= 1.8"
   spec.add_dependency "rubocop-ast", ">= 1.0.1"