chore: update dependency minitest to v6

Author: renovate-bot

spannerlib/wrappers/spannerlib-ruby/Gemfile

@@ -7,7 +7,7 @@ gemspec
 gem "rake", "~> 13.0"
 
 group :development, :test do
-  gem "minitest", "~> 5.0"
+  gem "minitest", "~> 6.0"
   gem "rake-compiler", "~> 1.0"
   gem "rspec", "~> 3.0"
   gem "rubocop", require: false