Rollup merge of rust-lang#134367 - WaffleLapkin:trait_upcasting_as_a_treat, r=compiler-errors

Stabilize `feature(trait_upcasting)`

This feature was "done" for a while now, I think it's finally time to stabilize it! Stabilization report: rust-lang#134367 (comment).
cc reference PR: rust-lang/reference#1622.

Closes rust-lang#65991 (tracking issue), closes rust-lang#89460 (the lint is no longer future incompat).

r? compiler-errors

Author: matthiaskrgr

coretests/tests/lib.rs

@@ -81,7 +81,6 @@
 #![feature(strict_provenance_atomic_ptr)]
 #![feature(strict_provenance_lints)]
 #![feature(test)]
-#![feature(trait_upcasting)]
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]
 #![feature(try_blocks)]