Rollup merge of #91535 - Aaron1011:stabilize-future-incompat, r=nagisa

Stabilize `-Z emit-future-incompat` as `--json future-incompat`

The FCP was completed in https://github.com/rust-lang/rust/issues/71249

compiler/rustc_session/src/session.rs

@@ -280,7 +280,7 @@ impl Session {
     }
 
     fn emit_future_breakage(&self) {
-        if !self.opts.debugging_opts.emit_future_incompat_report {
+        if !self.opts.json_future_incompat {
             return;
         }