Indicate a minimum rustc version in Cargo.toml?

[larsnaesbye]

I failed to compile rinex 0.6.1 and got error[E0658]: use of unstable library feature 'vec_retain_mut'.

I then updated Rust with rustup from 1.59 to 1.63 and the error went away. A little research shows that the feature was marked stable as of 1.61.

So perhaps we should indicate in Cargo.toml that the minimum Rust version is 1.61 so it can bail out early?