GeezerVison

This replaces the dependency of @actions/github with a more direct dependency on Octokit via @octokit/action - https://github.com/octokit/action.js.

@actions/github is on a very old version of Octokit and is preventing users of this action from using the latest GitHub APIs in their scripts:

• octokit.rest.actions not available #345
• Update octokit dependency to match online documentation #470
• AddSubIssue functions missing #519

Using Octokit in a more direct manner will allow this action to keep up date with dependencies in a more timely manner going forward.

This will likely break some user scripts, both from "expected" breaking changes from a multi-version bump of Octokit and unexpected breakages from new or different behavior in @octokit/action or this action that we're not correctly replicating from @actions/github. My hope is that integration tests will catch the more significant scenarios, but we should expect some bugs with the v8 version of this action once it's released - #556.