[ruby/rdoc] Drop support for Ruby 2.4

https://github.com/ruby/rdoc/commit/f480b970cc

lib/rdoc/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 @@ class TestRDocRDoc < RDoc::TestCase
       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