Bump required ruby version to 3.1

The code is using Ruby 3.1+ syntax, so we shouldn't let
it be installed by older ruby versions.

Author: byroot

importmap-rails.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
 
   spec.files = Dir["{app,lib}/**/*", "MIT-LICENSE", "Rakefile", "README.md"]
 
-  spec.required_ruby_version = ">= 2.7.0"
+  spec.required_ruby_version = ">= 3.1.0"
   spec.add_dependency "railties", ">= 6.0.0"
   spec.add_dependency "activesupport", ">= 6.0.0"
   spec.add_dependency "actionpack", ">= 6.0.0"