Add mutex_m as a dependency

Ruby 3.3 shows a deprecation message:

```
warning: mutex_m was loaded from the standard library, but will no
longer be 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.
```

This gem is needed by Rails 7.0 and 7.1, but has been removed in Rails
7.2 and higher.