This is a prerequisite for proper universe_domain support. It removes the code that makes direct calls to the Metadata Server (to determine GCE residence and to fetch Metadata-provided access tokens), and instead delegates to the more robust code in the google-cloud-env 2.0 gem.
Also drops support for Ruby 2.6 since google-cloud-env 2.0 also drops Ruby 2.6.
This is a prerequisite for proper universe_domain support. It removes the code that makes direct calls to the Metadata Server (to determine GCE residence and to fetch Metadata-provided access tokens), and instead delegates to the more robust code in the google-cloud-env 2.0 gem.
Also drops support for Ruby 2.6 since google-cloud-env 2.0 also drops Ruby 2.6.
Also closes #444.