Add mutex_m to gemspec for ruby 3.4

Fixes:
```
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.
```

kubeclient.gemspec

@@ -22,8 +22,9 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency 'bundler', '>= 1.6'
   spec.add_development_dependency 'rake', '~> 13.0'
-  spec.add_development_dependency 'minitest', '~> 5.15.0'
+  spec.add_development_dependency 'minitest', '~> 5.25'
   spec.add_development_dependency 'minitest-rg'
+  spec.add_development_dependency 'mutex_m', '~> 0.3'
   spec.add_development_dependency 'webmock', '~> 3.0'
   spec.add_development_dependency 'rubocop', '~> 1.3.0' # locked to minor so new cops don't slip in
   spec.add_development_dependency 'googleauth', '~> 1.3'