Multiple CI config updates

[timsutton]

A few CI configuration updates:

• specify Ruby versions loosely rather than very specific versions
• add macOS runners to the CI matrix (practically-speaking we almost exclusively run this gem on macOS so it would make sense to run CI on it, even if we don't have any integration tests at this point)
• fix a security warning about token permissions

This also deletes Gemfile.lock. It's not really a standard practice to check Gemfile.lock in for a gem, and the CI Ruby action was complaining our config in having it there and the concept of frozen deployment mode.