std: Change String::truncate to panic less

The `Vec::truncate` method does not panic if the length argument is greater than
the vector's current length, but `String::truncate` will indeed panic. This
semantic difference can be a bit jarring (e.g. #32717), and after some
discussion the libs team concluded that although this can technically be a
breaking change it is almost undoubtedly not so in practice.

This commit changes the semantics of `String::truncate` to be a noop if
`new_len` is greater than the length of the current string.

Closes #32717

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