Deprecate unnecessary constant in `GitHubMetadata::Client`

[ashmaroli]

I stumbled upon the realization that Faraday::ConnectionFailed is valid even in faraday-0.17 and older and that there is no need of testing for Faraday::Error::ConnectionFailed. The latter class is just an alias of the former.

Therefore, let us deprecate the GitHubMetadata::Client::FARADAY_FAILED_CONNECTION constant which should have ideally been a private_constant.