Make 15.11 minimum required version to upgrade to 16.0
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:
- Enforcing 15.0 to upgrade to 15.x - !6126 (merged)
- Enforcing 14.10 to upgrade to 15.0 - !6050 (merged)
- Enforcing 14.0 to upgrade to 14.x - !5398 (merged)