Drop support for dead old versions

Author: nobu

.github/workflows/ubuntu.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.6', '2.5', '2.4', '2.3', 'jruby-9.2.11.0', 'jruby-9.1.17.0', 'head' ]
+        ruby: [ 'head', '3.0', '2.7', '2.6', '2.5', 'jruby-9.2.11.0' ]
     steps:
     - uses: actions/checkout@master
     - name: Set up Ruby

.github/workflows/windows.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        ruby: [ '2.6', '2.5', '2.4' ]
+        ruby: [ 'head', '3.0', '2.7', '2.6', '2.5' ]
     steps:
     - uses: actions/checkout@master
     - name: Set up Ruby

fileutils.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |s|
 
   s.require_path = %w{lib}
   s.files = ["LICENSE.txt", "README.md", "Rakefile", "fileutils.gemspec", "lib/fileutils.rb"]
-  s.required_ruby_version = ">= 2.3.0"
+  s.required_ruby_version = ">= 2.5.0"
 
   s.authors = ["Minero Aoki"]
   s.email = [nil]