fix(publisher-github): remove octokit deprecation warnings

[b-zurg]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Implemented API Changes in order to remove deprecation warnings from github publisher.

Closes #1672

[malept]

Thanks for the pull request! I take it you've tested these changes manually?

All that's needed at this point is to fix the tests, which are unfortunately kind of fragile due to the module needing to access the network normally.

• Update these tests to reflect how the user agent is now specified:
  https://github.com/electron-userland/electron-forge/blob/1cfe04989c0976a1317c4b91c6467f7986ddda68/packages/publisher/github/test/github_spec.ts#L61-L63

https://github.com/electron-userland/electron-forge/blob/1cfe04989c0976a1317c4b91c6467f7986ddda68/packages/publisher/github/test/github_spec.ts#L68-L71

• It looks like the following tests need to be rewritten to not use mocks:

https://github.com/electron-userland/electron-forge/blob/1cfe04989c0976a1317c4b91c6467f7986ddda68/packages/publisher/github/test/github_spec.ts#L74-L86

They'll probably look closer to the user agent tests.

[mahnunchik]

@b-zurg could you please update the tests to have changes merged?

[b-zurg]

I'll take care of it tomorrow. Sorry it's taken me so long to finish this up.

[b-zurg]

I've made some of the necessary changes. I did a bit of investigation and it doesn't seem like an authenticate is necessary anymore for the GitHub class as all it did was set this's authentication token to the one passed in. So it didn't seem like the tests were necessary anymore.

I've had a lot of trouble getting bolt and tests running well, the dev cycle is very slow because of the extensive amount of tests, however I hope to get them running soon on my local env.

This PR is not ready for merge yet.

[malept]

For your purposes, you should be able to filter out most tests, by running:

yarn test --glob=test/github_spec.ts

It's something I need to add to CONTRIBUTING.md.