Rename internal logger

[tombruijn]

There may be some confusion between our logging feature (Appsignal::Logger) and our internal logger (Appsignal.logger).

Let's rename our internal logger so it's more clear what it's purpose is.

To do

• Rename Appsignal.logger to something more clear about its purpose, e.g. Appsignal.internal_logger.
  • This can be a patch release. Since Appsignal.logger is a private API, no one should be using it anyway.