Add limited support for bundled signatures of a later JDK version than currently executing

[uschindler]

As we do not yet support forking in the Maven/Gradle task and using Toolkits, it is not always possible to run checks against class files of a later version than the current Gradle/Maven runtime.

The code should in addition to #207 do the following:

• print a warning that scanning class files of later version than current runtime may not find all issues
• disable unknown signatures silently

[uschindler]

Actually this makes no sense, because if you run with a newer verson, the JDK related bundled sigantures are never available.