Drop support for Ruby 2.4

Author: nobu

.github/workflows/ubuntu.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ 'jruby-9.2.17.0', 'head', '3.0', '2.7', '2.6', '2.5', '2.4' ]
+        ruby: [ 'jruby-9.2.17.0', 'head', '3.0', '2.7', '2.6', '2.5' ]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby

.github/workflows/windows.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        ruby: [ 'head', '3.0', '2.7', '2.6', '2.5', '2.4' ]
+        ruby: [ 'head', '3.0', '2.7', '2.6', '2.5' ]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby

rdoc.gemspec

@@ -242,7 +242,7 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
     TODO.rdoc
   ]
 
-  s.required_ruby_version = Gem::Requirement.new(">= 2.4.0")
+  s.required_ruby_version = Gem::Requirement.new(">= 2.5.0")
   s.required_rubygems_version = Gem::Requirement.new(">= 2.2")
 
   s.add_development_dependency("gettext")

test/rdoc/test_rdoc_rdoc.rb

@@ -464,7 +464,7 @@ def test_remove_unparseable_CVE_2021_31799
       end
 
       assert_equal file_list, @rdoc.remove_unparseable(file_list)
-      assert_equal file_list, Dir.entries('.') - %w[. ..]
+      assert_equal file_list, Dir.children('.')
     end
   end