Remove `backup_dtype` from categorical logical types and bump pyspark version if necessary

[thehomebrewnerd]

With version 1.8.0, Koalas added support for the category dtype. We should bump our min Koalas version to 1.8.0 and update all of our categorical LogicalTypes to remove the backup_dtype attribute.

I think this will also allow us to remove the use of backup_dtype completely from the Woodwork code, including the usage in the LogicalType class.

This will also require updating several tests.

[tamargrey]

This may also allow us to remove LogicalType._get_valid_dtype and just use LogicalType.primary_dtype (though now that there's no backup dtype, maybe we should change the name of "primary_dtype")

[gsheni]

@tamargrey sounds good. We will be addressing this issue in the upcoming sprint.

[gsheni]

@rwedge ice-boxing this for now.