Upgrade to edition 2021 instead

[Xuanwo]

Hi, futures-rustls is still using edition 2018 and doesn't have an MSRV yet. How about aligning it with the same version as rustls, which uses Edition 2021 and an MSRV of 1.71?

I'm willing to help with this change.

[quininer]

Given that tokio-rustls is already the 2021 edition, it seems like a good idea to update it.