[ruby/io-console] No longer support dead versions

https://github.com/ruby/io-console/commit/e9478509e9

ext/io/console/io-console.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.summary = "Console interface"
   s.email = "nobu@ruby-lang.org"
   s.description = "add console capabilities to IO instances."
-  s.required_ruby_version = ">= 2.4.0"
+  s.required_ruby_version = ">= 2.6.0"
   s.homepage = "https://github.com/ruby/io-console"
   s.metadata["source_code_url"] = s.homepage
   s.authors = ["Nobu Nakada"]