Bump up the required ruby version to 2.7

Author: nobu

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
       engine: cruby-truffleruby
-      min_version: 2.6
+      min_version: 2.7
 
   build:
     needs: ruby-versions

etc.gemspec

@@ -40,5 +40,5 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
   spec.extensions    = %w{ext/etc/extconf.rb}
 
-  spec.required_ruby_version = ">= 2.6.0"
+  spec.required_ruby_version = ">= 2.7.0"
 end