modules/git-workflow-github.md: GitHub Lab tool discontinued

[OleksiyRudenko]

GitHub Lab https://lab.github.com/install?org=githubtraining&course=managing-merge-conflicts (see https://github.com/kottans/git-course/blob/master/modules/git-workflow-github.md#d7-approve-and-merge-pull-request:~:text=Install-,GitHub%20lab%20tool,-and%20watch%20a and GitHub Learning Lab - practical courses ) is not available anymore.

Suggested fix: remove or replace with a similar training material.