Supports #133, #155, #179
This updates the github authenticated Octokit client available in this action to version 5 of the Octokit client. This change allows users to script using the latest released API.

Since this is a major version update and includes breaking changes, this will come with a v5 release of this action.

The primary breaking change is that REST methods must use github.rest.* instead of github.*. I've added a note to the README for this change and updated our examples.