Enabling native GitHub auto-merge feature for Pull Requests

[achauphan]

Description

One of the common features of AutoTester1 was its Pull Request auto-merge capability that was controlled with a label and dependent upon all checks being passed.

Since AutoTester1 is no longer with us, we will be enabling the native GitHub auto-merge feature.

How it works

From GitHub's official documentation:

To enable auto-merging for a PR with GitHub, you must scroll down to the checks box and click "Enable auto-merge".

With this change, the AutoTester1 auto-merge label "AT: AUTOMERGE" will be removed.