Remove support for Ruby 3.0

Update Ruby version in gemspec file

Author: MuriloDalRi

rubocop-govuk.gemspec

@@ -11,7 +11,7 @@ Gem::Specification.new do |spec|
 
   spec.files         = Dir["config/**/*", "*.md"]
 
-  spec.required_ruby_version = ">= 3.0"
+  spec.required_ruby_version = ">= 3.1"
 
   spec.add_development_dependency "rake", "~> 13"