Remove `#![feature(label_break_value)]` as it's been stabilized

[kkysen]

#![feature(label_break_value)] has now been stabilized in 1.65 (see rust-lang/rust#99332, https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#break-from-labeled-blocks).

[GPHemsley]

1.65 was branched from master on 2022-09-16, so probably want to update the toolchain at least that far first?