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

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

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

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

r? ```@Urgau```

Author: fmease

triagebot.toml

@@ -1424,3 +1424,6 @@ compiletest = [
 # Enable `@rustbot note` functionality
 # Documentation at: https://forge.rust-lang.org/triagebot/note.html
 [note]
+
+[behind-upstream]
+days-threshold = 14