Rollup merge of #141071 - xizheyin:behind-upstream, r=Urgau

Enable [behind-upstream] triagebot option for rust-lang/rust

After testing in [rustc-develop-guide](#2384 (comment)), we can turn on `behind-upstream` here.

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

r? ```@Urgau```

Author: fmease