Switch to 2021 edition of Rust

[SebastianSchildt]

This upgrades RUST edition to 2021 and also explicitly enables the new dependency resolver

This is supported since RUST 1.56, and usually we are building with much newer versions anyway

https://blog.rust-lang.org/2021/05/11/edition-2021.html https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html

The "next" release will be the 2024 edition, so guess switching to 2021 is pretty sensible now https://blog.rust-lang.org/2023/12/15/2024-Edition-CFP.html

Will close #328

[erikbosch]

If we update a Cargo.toml, should we then request that PR creator also have done a cargo update so that Cargo.lock gets updated?

As I understand it does not affect builds, but Dependabot use it when scanning for vulnerabilities. As of today we update it as part of creating releases, but do we see a need to update also in other cases like when a Cargo.toml is updated?

[SebastianSchildt]

I updated Cargo.lock now. I think it is good practice when touching Cargo files

[argerus]

Do you see any problems @argerus

Nope, looks good to me.