CI: use JDK8 for all checks
CI: use JDK8 for all checks

Since Checkstyle now requires JDK8 to run, make them all use JDK8. Also
best since we'll eventually transition to it as the required version.

workcraft/workcraft,tuura/workcraft,danilovesky/workcraft,danilovesky/workcraft,jrbeaumont/workcraft,danilovesky/workcraft,jrbeaumont/workcraft,tuura/workcraft,tuura/workcraft,workcraft/workcraft,jrbeaumont/workcraft,workcraft/workcraft,workcraft/workcraft,danilovesky/workcraft