Move platform jobs to JDK 11.

 - test everything on JDK 11 which can run on JDK 11
 - leave the rest in a JDK 8 section of the platform job for
   now (makes it easy to see what still needs work)
 - remove relict from travis times:
   can't find any references to -Dvanilla.javac.exists=true

other jobs:

 - bump java debugger tests from [11, 17] to [11, 21]
 - bump build-tools job from [11, 17] to [11, 21], isolate tests
   which can't run on JDK 21 yet and run them on 17