Establish Minimum Supported Rust Version (MSRV)

[alilleybrinker]

We should be clear about how old a Rust version can be used to compile both the gitoid and omnibor crates. This will be dependent on a few things:

• The MSRV of the crates we depend on.
• What versions of Rust are packaged in the system package managers of popular operating systems.
• The features we need to use vs. ones we can find older alternatives for.

The second criteria isn't necessarily a blocker to a higher MSRV, but it ought to be part of the consideration. If we want the Rust implementation to be the reference and/or basis implementation for other languages, then part of that will include wanting it to be easy to package and distribute. This is easiest if it works with a version of the language that's widely available, versus always tracking the latest stable release.

Once we've established an MSRV, we should also establish MSRV testing in CI.

[alilleybrinker]

We now explicitly document that there's no MSRV. It's something good to commit to later, but things are too in flux now for this to make sense.