Updated YANG Module Revision Handling

This document refines the RFC 7950 module update rules. It specifies a new YANG module update procedure that can document when non- backwards-compatible changes have occurred during the evolution of a YANG module. It extends the YANG import statement with a minimum revision suggestion to help document inter-module dependencies. It provides guidelines for managing the lifecycle of YANG modules and individual schema nodes. This document updates RFC 7950, RFC 6020, RFC 8407 and RFC 8525.