bump required ruby version to 2.7

Author: khiga8

.github/workflows/test.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         ruby:
-          - '2.6'
+          - '2.7'
 
     steps:
     - uses: actions/checkout@v2

rubocop-rails-accessibility.gemspec

@@ -11,7 +11,7 @@ Gem::Specification.new do |spec|
   spec.summary = "Custom extension for RuboCop."
   spec.homepage = "https://github.com/dummy/rubocop-rails-accessibility"
   spec.license = "MIT"
-  spec.required_ruby_version = ">= 2.6.0"
+  spec.required_ruby_version = ">= 2.7.0"
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage