upgrade to gradle 6.9.x (prep for gradle 7.x)

[keturn]

Gradle 7 has some features Terasology is interested in (e.g. version catalogs, Java 17 support). Since the MovingBlocks/✳ libraries are often used in composite builds with Terasology, it seems like a good idea to keep their gradle versions from drifting too far apart.

I propose upgrading all the libraries to the latest in the 6.x line (currently 6.9.0) and fixing whatever deprecation warnings crop up there. That should give us a smooth path to then adopting Gradle 7.

• upgrade Terasology repo itself
• upgrade to gradle 6.9.x (prep for gradle 7.x) JNBullet#13
• upgrade to gradle 6.9.x (prep for gradle 7.x) TeraNUI#57
• upgrade to gradle 6.9.x (prep for gradle 7.x) gestalt#116
• upgrade to gradle 6.9.x (prep for gradle 7.x) CrashReporter#47