Rollup merge of #99573 - tbodt:stabilize-backtrace, r=yaahc

Stabilize backtrace

This PR stabilizes the std::backtrace module. As of #99431, the std::Error::backtrace item has been removed, and so the rest of the backtrace feature is set to be stabilized.

Previous discussion can be found in #72981, #3156.

Stabilized API summary:
```rust
pub mod std {
    pub mod backtrace {
        pub struct Backtrace { }
        pub enum BacktraceStatus {
            Unsupported,
            Disabled,
            Captured,
        }
        impl fmt::Debug for Backtrace {}
        impl Backtrace {
            pub fn capture() -> Backtrace;
            pub fn force_capture() -> Backtrace;
            pub const fn disabled() -> Backtrace;
            pub fn status(&self) -> BacktraceStatus;
        }
        impl fmt::Display for Backtrace {}
    }
}
```

`@yaahc`

compiler/rustc_middle/src/lib.rs

@@ -26,7 +26,6 @@
 #![feature(allocator_api)]
 #![feature(array_windows)]
 #![feature(assert_matches)]
-#![feature(backtrace)]
 #![feature(box_patterns)]
 #![feature(core_intrinsics)]
 #![feature(discriminant_kind)]