chore: Add mutex_m gem for Rails 7.0 compatibility with 3.4

Starting from Ruby 3.4, mutex_m has been changed from a default gem to a
bundled gem.In systems using Bundler (such as Rails applications), the
gem needs to beexplicitly listed in the Gemfile and installed.This
change ensures that the mutex_m library is properly availablein Ruby 3.4
environments.

Author: hatsu38

gemfiles/Gemfile.activerecord-7.0.x

@@ -6,3 +6,5 @@ gem "pg", "~> 1.5.9"
 gem "mysql2", "~> 0.5.6"
 gem "sqlite3", "~> 1.4"
 gem "logger"
+gem "mutex_m"
+gem "bigdecimal"