Enable [behind-upstream] triagebot option

[xizheyin]

Let's try running it on a repository first.

Doc: https://forge.rust-lang.org/triagebot/behind-upstream.html

r? @Urgau

cc @Kobzol

[Kobzol]

Tbh it's likely a bit useless in this specific repo, because it doesn't run almost any tests and I doubt anyone had issues with an old master here 😆 But no harm in enabling it to test it, that's for sure.

[xizheyin]

Haha, yes, Urgau told me to try it out by enabling it on a less important repository first.

[Urgau]

Yeah, this repo is probably a bit too low volume for the feature to be really useful, but as @Kobzol said, no harm in enabling it for testing.