Bump MSRV to 1.56 to get access to Rust 2021 crates

[mgeisler]

I noticed in #1864 that mdBook does not yet use the Rust 2021 edition.
This 2021 edition was made stable in the 1.56 release on 2021-10-21.
The previous MSRV (1.54) was released on 2021-07-29.

[ehuss]

Is there something specific that needs this to be updated?

I'm a bit confused to the relationship to #1062, as that is 3 years old.

There is #1831 to update the actual edition, but the PR currently has some failures. If someone else wants to revive it, I think that would be fine. But otherwise I'm not sure if I'd bother updating the version without a need.

[mgeisler]

Is there something specific that needs this to be updated?

I'm a bit confused to the relationship to #1062, as that is 3 years old.

Hi @ehuss, I'm sorry, I don't know what I was copying before... I wanted to refer to #1864 and ended up copying something else. I've updated it now in the comment and will fix it in the commit too.

Thanks for the pointer to #1831, I will be happy to see what needs to be fixed there.

[mgeisler]

The same change (and more) is being made in #1831.

[mgeisler]

The edition was eventually bumped in #1887.