Swap Default Package Manager from yarn to npm

[ksylvest]

Context

This changes the default package manager used when bootstrapping projects form yarn to npm.

See: #217

[ksylvest]

Unclear why CI is not happy. Running locally I confirmed the Ruby 3.2 / Rails 7.0 worked fine via:

rbenv local 3.2.6
bundle exec appraisal rails_7_0 rake test

# Running:

.....................

Finished in 72.543269s, 0.2895 runs/s, 2.4124 assertions/s.
21 runs, 175 assertions, 0 failures, 0 errors, 0 skips

Possibly a flakey run?