R1:0f1acb6cef1d

Update GitHub API calls to use "Authorization" header instead of "access_token" URI parameter

Summary: Fixes T13485. GitHub has deprecated the "access_token" URI parameter for API authentication. Update to "Authorization: token ...".

Test Plan: Linked and unlinked a GitHub account locally.

Maniphest Tasks: T13485

Differential Revision: https://secure.phabricator.com/D20964