btreemap doesn't work with Java 17

[brunnsbe]

The code uses sun.misc.Unsafe which isn't supported anymore in Java 17: https://github.com/batterseapower/btreemap/blob/ba6f9ce32e99ccb9b4e965f1e634e5df8bd6b838/src/main/templates/uk.co.omegaprime.btreemap/%7B%7BKV_%7D%7DNode.java#L64

The problem can be fixed by replacing it with jdk.internal.misc.Unsafe.objectFieldOffset but it would be better to altogether get rid of the usage of Unsafe.