Rename existing ensembler implementation from StackedEnsemblers to SklearnStackedEnsemblers

[christopherbunn]

Renaming existing ensembler component to clarify it is sklearn based and to differentiate it from the new ensembler component.

[codecov]

Codecov Report

Merging #2578 (0731e22) into main (455210b) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #2578   +/-   ##
=====================================
  Coverage   99.9%   99.9%           
=====================================
  Files        295     295           
  Lines      27055   27055           
=====================================
  Hits       27012   27012           
  Misses        43      43           

Impacted Files	Coverage Δ
evalml/pipelines/__init__.py	100.0% <ø> (ø)
evalml/pipelines/pipeline_base.py	98.3% <ø> (ø)
evalml/tests/automl_tests/test_evalml_algorithm.py	100.0% <ø> (ø)
evalml/utils/gen_utils.py	99.6% <ø> (ø)
evalml/pipelines/components/__init__.py	100.0% <100.0%> (ø)
evalml/pipelines/components/ensemble/__init__.py	100.0% <100.0%> (ø)
...mponents/ensemble/sklearn_stacked_ensemble_base.py	100.0% <100.0%> (ø)
...ts/ensemble/sklearn_stacked_ensemble_classifier.py	100.0% <100.0%> (ø)
...nts/ensemble/sklearn_stacked_ensemble_regressor.py	100.0% <100.0%> (ø)
evalml/pipelines/utils.py	99.2% <100.0%> (ø)
... and 8 more

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[chukarsten]

LGTM.