Bring GryoRegistrator to the forefront and deprecate GryoSerializer

At the next major release (3.3.0), we should update the documentation and show the safer model for using Kryo with SparkGraphComputer.

spark.serializer=KryoSerializer
spark.kryo.registrator=GryoRegistrator

GryoSerializer will simply be deprecated (and thus, backwards compatible).

When deprecating GryoSerializer we should update the priority for the UnshadedKryoShimService as well. Also, in the documentation, we will have to explain that people should implement SerializerShim instead of Serializer if they want their custom classes to work moving forward. The change for them is as simple as replacing implements Serializer with implement SerializerShim and handling respective imports.