Group's use of uasort returns booleans, switch to spaceship operator.

Will commit to 2.0.x for now, but might backport to the previous version if 2.0.x doesn't get released "soon".

In GroupPermissionHandler::sortPermissions() we are returning the result of a greater-than check, i.e. a boolean. Recent version of PHP no longer like this and expect either -1, 0 or 1. We can use the spaceship operator <=> to achieve exactly that.