Bump up the required ruby version

io-wait 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.

Author: nobu

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
       engine: cruby
-      min_version: 2.6
+      min_version: 3.0
 
   test:
     needs: ruby-versions

io-wait.gemspec

@@ -10,6 +10,7 @@ Gem::Specification.new do |spec|
   spec.description   = %q{Waits until IO is readable or writable without blocking.}
   spec.homepage      = "https://github.com/ruby/io-wait"
   spec.licenses      = ["Ruby", "BSD-2-Clause"]
+  spec.required_ruby_version = Gem::Requirement.new(">= 3.0")
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage