Auto merge of #77853 - ijackson:slice-strip-stab, r=Amanieu

Auto merge of #77853 - ijackson:slice-strip-stab, r=Amanieu

Stabilize slice::strip_prefix and slice::strip_suffix

These two methods are useful.  The corresponding methods on `str` are already stable.

I believe that stablising these now would not get in the way of, in the future, extending these to take a richer pattern API a la `str`'s patterns.

Tracking PR: #73413.  I also have an outstanding PR to improve the docs for these two functions and the corresponding ones on `str`: #75078

I have tried to follow the [instructions in the dev guide](https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#stabilization-pr).  The part to do with `compiler/rustc_feature` did not seem applicable.  I assume that's because these are just library features, so there is no corresponding machinery in rustc.