Use a more recent Rust toolchain

[RyanGlScott]

The Rust 2024 language edition is scheduled to be finalized real soon now as part of the 1.85.0 release of the Rust toolchain. We should update mir-json to target a nightly that includes Rust 2024 language edition support.

@spernsteiner has made some amount of progress towards this goal in this branch, but there is almost certainly more work that needs to be done.

[RyanGlScott]

I've retitled this to "a more recent Rust toolchain". I chose Rust 2024 as it is an arbitrary milestone in the future, but what is more important is having support for sufficiently new Rust code (for some definition of "sufficently new").

[spernsteiner]

Updating to the latest edition is a good idea anyway. Once the new edition is stable, cargo new will use it by default, and some existing projects will gradually migrate, making them difficult to build with mir-json even if they don't use any new APIs or language features. We've run into trouble with this in the past.

But I believe the 2024 edition is supported in the current nightlies, so any toolchain version we're likely to upgrade to will already support the edition.