Allow running on Ruby 3

Author: elia

lib/solidus_dev_support/templates/extension/extension.gemspec.tt

@@ -17,7 +17,7 @@ Gem::Specification.new do |spec|
   spec.metadata['source_code_uri'] = '<%= gemspec.metadata["source_code_uri"] %>'
   spec.metadata['changelog_uri'] = '<%= gemspec.metadata["changelog_uri"] %>'
 
-  spec.required_ruby_version = Gem::Requirement.new('~> 2.5')
+  spec.required_ruby_version = Gem::Requirement.new('>= 2.5', '< 4')
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.