Move function drop_unknown_references from poc to be directly under utils

[npatki]

Problem Description

Currently the drop_unknown_references is available as a utility function under a submodule called poc

from utils import poc

cleaned_data = poc.drop_unknown_references(data=data, metadata=metadata)

The poc module is meant to house functions that are needed to successfully complete a proof-of-concept using public SDV (HMASynthesizer). However, it is now clear to us that even those users who are not doing proof-of-concept may require this function.

For example, a customer using SDV Enterprise may still need drop_unknown_references if their original data does not contain referential integrity.

Expected behavior

Move this function to be under utils instead of poc.

import utils

cleaned_data = utils.drop_unknown_references(data=data, metadata=metadata)

Additional context

For backwards compatibility, we could still allow the function to work under poc but show a FutureWarning explaining to users that the location has changed.

FutureWarning: Please access the 'drop_unknown_references' function directly from the utils module 
instead of utils.poc.