Declare minimum Rust version

[zanieb]

per astral-sh/uv#2442 (comment) the lack of this can seemingly cause problems?

[Eh2406]

I am happy to set the relevant field so that our users get better error messages. But I do not want anyone contribute to this project to have to wait on improvements in language. However we set up our MSRV and associated CI, it has to be our policy (and mechanically very easy to) change the MSRV to use new functionality.