Set rust-version for workspace members

[sorairolake]

The edition of all crates in this repository are Rust 2021 (requires Rust 1.56.0 or later). So we can set the rust-version field for the Cargo.toml file. This makes it easier to check the crate's MSRV.

I didn't feel the need to set this field for crates which aren't published on crates.io, so I didn't set the field for codegen and explorer-thumbnailer crates.

[DJMcNab]

Welcome @sorairolake!

This PR has arrived at an interesting time for the project, as we're going through a relicensing. To be able to accept this contribution, we're going to need you to agree to that - see #838

One other note, the badge in the README lists the MSRV as 1.67.1. I'd probably set the MSRV to that to start with.

Thank you

[sorairolake]

#838 (comment)

[sorairolake]

@DJMcNab The MSRV of usvg is Rust 1.65.0 (checked with cargo-msrv), should this also be changed to Rust 1.67.1?

[DJMcNab]

When we move to Linebender CI (see linebender/rfcs#5), we will probably want to have a unified MSRV. But since this isn't being checked, just specifying the actual reality is fine.

Thank you!