Replace Common::BitCast with std::bit_cast

[Pokechu22]

Now that we're using C++20, we can use std::bit_cast instead. See #10737 for a bit of other context.

[Zopolis4]

I'm not sure about the other builders, but we can (and should) definitely update the debian builders to bullseye (preferably sid), and that would solve those build failures.