Bump rust edition to 2021

[Jakuje]

Turns out we used in one of the last changes a feature from edition 2021, which current warns in main like this:

warning: panic message contains an unused formatting placeholder
    --> cryptoki/tests/basic.rs:1043:63
     |
1043 |             "Only 3.0 and 3.2 versions are expected but got 3.{minor}"
     |                                                               ^^^^^^^
     |
     = note: this message is not used as a format string when given without arguments, but will be in Rust 2021
     = note: `#[warn(non_fmt_panics)]` on by default
help: add the missing argument
     |
1043 |             "Only 3.0 and 3.2 versions are expected but got 3.{minor}", ...
     |                                                                       +++++
help: or add a "{}" format string to use the message literally
     |
1043 |             "{}", "Only 3.0 and 3.2 versions are expected but got 3.{minor}"
     |             +++++

This made me wondering why we are on the 2018 these days and it looks like the only thing to fix this is a bump of the rust edition to 2021.

#262 (comment)

I did not notice any side effects locally.

[Jakuje]

I would consider bumping to 2024, but last time we tried to do this in kryoptic, the bindgen 0.71 was not ready.

[wiktor-k]

LGTM 👍 too bad 2024 is not available but we can always revise.