Revert stabilization of trait_upcasting feature
Reverts #118133
This reverts commit 6d2b84b3ed7848fd91b8d6151d4451b3103ed816, reversing changes made to 73bc12199ea8c7651ed98b069c0dd6b0bb5fabcf.
The feature has a soundness bug:
* #120222
It is unclear to me whether we'll actually want to destabilize, but I thought it was still prudent to open the PR for easy destabilization once we get there.
a787232abb