Clarify preferred Cloud Foundry CLI version (or lack of a preference)

[nick-jones-gov]

In order to help new users of cloud.gov get set up, it would be helpful if the documentation for setting up the command line mentioned which version of Cloud Foundry CLI users should install (or if it makes no difference which version they install). Currently, the Cloud Foundry documentation mentions both v6 and v7 and new users (such as myself!) may not be sure if there are any version requirements for cloud.gov's use of Cloud Foundry.

Thanks for your help!