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

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.