[ruby/io-nonblock] Bump up the required ruby version

io-nonblock became a default gem at ruby 3.0.
Even it can be installed on earlier versions, but the standard library
will be loaded instead of the installed gem.

ruby/io-nonblock@c86d0d37af

Author: nobu

ext/io/nonblock/io-nonblock.gemspec

@@ -8,7 +8,7 @@ Gem::Specification.new do |spec|
   spec.description   = %q{Enables non-blocking mode with IO class}
   spec.homepage      = "https://github.com/ruby/io-nonblock"
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
-  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+  spec.required_ruby_version = Gem::Requirement.new(">= 3.0")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage