Rollup merge of #139532 - bjoernager:master, r=tgross35

Update `u8`-to-and-from-`i8` suggestions.

`u8::cast_signed` and `i8::cast_unsigned` have been stabilised, but `i8::from_ne_bytes` et al. still suggest using `as i8` or `as u8`.

Author: matthiaskrgr