Update `rbush`

[jjspace]

v4 of rbush switched to ES modules only so it doesn't work while we still support using CesiumJS in a CommonJS project.

This issue is just to track why we pinned it, see for more context: https://github.com/CesiumGS/cesium/issues/12191

Alternatively we may want to look into replacing this functionality with a different package that can be kept up to date or roll our own. This is primarily used for entity clustering.