Find the minimum supported Rust version

[scott-wilson]

It is generally a good idea to note what is the version of Rust that is supported by the project. We could either say "the current compiler is now the minimum (1.66 as of writing), and go from there", "the current is always the minimum supported", or go through the different versions to find out where the tests and compiling breaks, then call the version after that the latest supported version.