Deprecate Scikit-Learn based Ensembling Component

[christopherbunn]

As part of our implementation of our new ensembler component, we will need to deprecate the current scikit-learn based StackingEnsembler. Once we finish the new ensembler component, it will be used alongside the scikit-learn version during an ensembling AutoML search. However, because the new ensembler component's will contain the same functionality of the scikit-learn based ensembler, there isn't a need to keep both. This issue tracks deprecating the scikit-learn based ensembler.

Current implementation plan:

• Keep both ensemblers in AutoML search, but raise a DeprecationWarning during search that the sklearn version will not be included in search during the next release.
• Remove the scikit-learn based ensembler from AutoML search, but still retain
  • This will allow users to create new pipelines with this component manually if necessary
• Eventually, remove the scikit-learn based ensembler component from EvalML completely