Support ruby 3.2 fix

[lwoodson]

First two commits were reviewed in https://github.com/joshuaflanagan/serverless-ruby-package/pull/50

Last commit is new and changes the following:

• Moves the build-gems.sh script from the demo-service into the project root and ensures it is distributed with the npm package when released
• Changes the docker command to execute the build to invoke the build-gems.sh script from bash in its new location.
• Modifies the integration test to locally package and install the plugin into the demo-service at the start of each run