fix: revert removal of mutex_m

[vburzynski]

including mutex_m as a dependency is sometimes necessary

>> BUNDLE_GEMFILE=/Users/valerie/dev/thoughtbot/factory_bot/gemfiles/6.1.gemfile bundle exec rspec
/Users/valerie/.asdf/installs/ruby/3.4.6/lib/ruby/gems/3.4.0/gems/activesupport-6.1.7.10/lib/active_support/notifications/fanout.rb:3: 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.

An error occurred while loading spec_helper. - Did you mean?
                    rspec ./spec/spec_helper.rb

Failure/Error: require "active_support/deprecation"

LoadError:
  cannot load such file -- mutex_m
# ./lib/factory_bot.rb:5:in '<top (required)>'
# ./spec/spec_helper.rb:7:in '<top (required)>'
No examples found.


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0 examples, 0 failures, 1 error occurred outside of examples