[enhancement] Update deprecated devDependency github to @octokit/rest

[bjornstar]

PR checklist

• Addresses an existing issue: fixes devDependency 'github' has been renamed to '@octokit/rest'  #4672

Overview of change:

The github package has been deprecated -- https://git.io/vNB11

Is there anything you'd like reviewers to focus on?

The auth token is not necessary for public readonly access.

There were a few deprecated methods and properties that have been replaced by their modern equivalent.

I tested it locally and got the following output:

$ node ./scripts/generate-changelog.js
Getting commits 5.16.0..master

---- formatted changelog entries: ----
- [chore] Update mocha from v3.2.0 to v6.1.3 (#4669)
- [enhancement] Update dependency js-yaml from ^3.13.0 to ^3.13.1 (#4663)

---- PRs with missing changelog entries: ----
- Update docs config file
- VSCode plugin link updated (#4670)

---- thanks ----
Thanks to our contributors!
- Adi Dahiya
- Bjorn Stromberg
- Vitaliy Agoshkov

CHANGELOG.md entry:

[enhancement] Update deprecated devDependency github to @octokit/rest

[adidahiya]

nice! thanks @bjornstar

[adidahiya]

Updated the wiki to reflect that we no longer need the auth token.

[adidahiya]

Unfortunately I now get "HttpError: API rate limit exceeded" with this approach. So I think we will need to bring back authentication (which has a higher rate limit)