Auto merge of #106627 - Ezrashaw:no-e0711-without-staged-api, r=Mark-Simulacrum

fix: don't emit `E0711` if `staged_api` not enabled

Fixes #106589

Simple fix, added UI test.

As an aside, it seems a lot of features are susceptible to this, `E0711` stands out to me because it's perma-unstable and we are effectively exposing an implementation detail.

Author: bors