Rollup merge of #135841 - oli-obk:push-qxlnokwrkkym, r=compiler-errors

Reject `?Trait` bounds in various places where we unconditionally warned since 1.0

fixes #135730
fixes #135809

Also a breaking change, so let's see what crater says.

This has been an unconditional warning since *before* 1.0

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -102,8 +102,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 seen_sized_unbound = true;
                 continue;
             }
-            // There was a `?Trait` bound, but it was not `?Sized`; warn.
-            self.dcx().span_warn(
+            // There was a `?Trait` bound, but it was not `?Sized`
+            self.dcx().span_err(
                 unbound.span,
                 "relaxing a default bound only does something for `?Sized`; \
                 all other traits are not bound by default",