Add mutex_m to Gemfile for Ruby >= 3.4

Context:

warning: mutex_m was loaded from the standard library, but is not part of the default gems starting from Ruby 3.4.0.
You can add mutex_m to your Gemfile or gemspec to silence this warning.
/.rvm/rubies/ruby-3.4.7/lib/ruby/3.4.0/bundled_gems.rb:82:in 'Kernel.require': cannot load such file -- mutex_m (LoadError)

Author: wasifhossain

Gemfile

@@ -100,6 +100,7 @@ group :development, :test do
 
     if RUBY_VERSION >= '3.4.0'
       gem 'ostruct', require: false
+      gem 'mutex_m', require: false
     end
   end
   if version <= '5.0'