Stabilize default_alloc_error_handler Tracking issue: #66741 This turns `feature(default_alloc_error_handler)` on by default, which causes the compiler to automatically generate a default OOM handler which panics if `#[alloc_error_handler]` is not provided. The FCP completed over 2 years ago but the stabilization was blocked due to an issue with unwinding. This was fixed by #88098 so stabilization can be unblocked. Closes #66741