[ruby/fileutils] Drop support for dead old versions

https://github.com/ruby/fileutils/commit/4526148c67

lib/fileutils.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |s|
 
   s.require_path = %w{lib}
   s.files = ["LICENSE.txt", "README.md", "Rakefile", "fileutils.gemspec", "lib/fileutils.rb"]
-  s.required_ruby_version = ">= 2.3.0"
+  s.required_ruby_version = ">= 2.5.0"
 
   s.authors = ["Minero Aoki"]
   s.email = [nil]