Currently a mongod fails to startup if there exists any compound wildcard index and the FCV is downgraded below 7.0. We should only ban any new creation of a CWI if FCV < 7.0 but allow CWI's continued use and mongod should start successfully.

Historically, if FCV is downgraded, we choose to fail a start of a mongod with the existence of a new index type in the collection. In order to help customer get out of the situation that the mongod server cannot be restarted after FCV is downgraded and a CWI is not removed, we should still allow the index creation on start up even if FCV < 7.0. Only starting a mongod with the binary version < 7.0 should fail.