Drop Ruby 3.1 support

... as it is already EOL

Author: mame

.github/workflows/ruby.yml

@@ -13,7 +13,7 @@ jobs:
     uses: ruby/actions/.github/workflows/ruby_versions.yml@master
     with:
       engine: cruby
-      min_version: 3.1
+      min_version: 3.2
 
   build:
     needs: ruby-versions

error_highlight.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/ruby/error_highlight"
 
   spec.license       = "MIT"
-  spec.required_ruby_version = Gem::Requirement.new(">= 3.1.0.dev")
+  spec.required_ruby_version = Gem::Requirement.new(">= 3.2.0")
 
   spec.files = Dir.chdir(File.expand_path(__dir__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{\A(?:test|spec|features)/}) }