Handle com.google.cloud API libraries already added to projects when switching Java version

[chanseokoh]

This is closely related to #2067, so read it first.

It has been speculated that the com.google.cloud API libraries don't work with Java 7. At least, it is known that they don't work on the App Engine Standard runtime 7. It'd be nice if we alert users that the libraries won't work when they downgrade the project JRE to 7 (from 8 or higher).