Fix missing const for inherent pointer `replace` methods
`ptr::replace` (the free fn) is already const stable. However, there are inherent convenience methods on `*mut T` and `NonNull<T>`, allowing you to write eg. `unsafe { foo.replace(bar) }` where `foo` is `*mut T` or `NonNull<T>`.
It seems const was never added to the inherent method (likely oversight), so this PR adds it.
I don't believe this needs another[^1] FCP as the inherent methods are already stable and `ptr::replace` is already const stable, so this adds no new API.
Original tracking issue: #83164
`ptr::replace` constified in #83091
`ptr::replace` const stabilized in #130954
[^1]: `const_replace` FCP completed: https://github.com/rust-lang/rust/issues/83164#issuecomment-2385670050
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