handle bors-ng shutdown

[duckinator]

bors-ng has been deprecated. we should either self-host an instance, or move to an alternative (e.g. GitHub's Merge Queues).

EDIT: It seems that, as of November 7 2023, the public bors-ng instance has been down for multiple days.

[duckinator]

apparently Merge Queues aren't available for users, only organizations.

for some reason.

[nbraud]

Yeap, it is the worst. :/

[duckinator]

It's also very bizarre given that they published a post on the GitHub blog back in July that was titled "GitHub merge queue is generally available"... only to go on to mention that it's specific to "teams".

(As best I can tell, when they say team they mean organization? I'm not sure why they use the wrong term for their own product...)