Make 15.11/6.11 minimum required version to upgrade to 16.0/7.0

According to our upgrade policy, to jump to first release of a major version series, users must already be on the last minor version of the previous major series. Enforce this for 16.0 also.

Previous examples:

• Only permit 15.0 upgrade from 14.10 - !2562 (merged)
• Only permit upgrades from 4.12 - !2029 (merged)