adjust transmute const stabilization version

With 1.46, this became callable only in `const`/`static` items.

Only since 1.56 is this callable in `const fn`: [changelog](https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1560-2021-10-21)

Also see [Zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/transmute.20const.20fn.20stabilization).