Document the min. required version of the aws cli

[G-Rath]

We require at least v2.3.7 as that's when support for reason landed - I don't think we need to do anything specific for this as the cli is pretty easy update so it's reasonable to expect people to be on at least that version, but we might as well document it so make it easier for folks to debug.

(this also applies to our internal documentation)

[G-Rath]

though thinking about it, it could be useful to have a check_system_requirements class method that checks for this + the session-manager-plugin which we could then call in places like rake tasks as a guard rather than having to handle that on every project 🤔