Rename date_index to time_index

[freddyaboulton]

Pull Request Description

Fixes #3074

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After creating the pull request: in order to pass the release_notes_updated check you will need to update the "Future Release" section of docs/source/release_notes.rst to include this pull request by adding :pr:123.

[codecov]

Codecov Report

Merging #3137 (6816f2a) into main (e31c88f) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #3137   +/-   ##
=====================================
  Coverage   99.7%   99.7%           
=====================================
  Files        318     318           
  Lines      30756   30756           
=====================================
  Hits       30652   30652           
  Misses       104     104           

Impacted Files	Coverage Δ
evalml/automl/utils.py	100.0% <ø> (ø)
...omponents/estimators/regressors/arima_regressor.py	100.0% <ø> (ø)
.../transformers/preprocessing/datetime_featurizer.py	100.0% <ø> (ø)
.../pipelines/time_series_classification_pipelines.py	100.0% <ø> (ø)
...valml/pipelines/time_series_regression_pipeline.py	100.0% <ø> (ø)
evalml/preprocessing/utils.py	100.0% <ø> (ø)
...ts/automl_tests/parallel_tests/test_automl_dask.py	100.0% <ø> (ø)
.../automl_tests/test_automl_search_classification.py	100.0% <ø> (ø)
...data_checks_tests/test_ts_parameters_data_check.py	100.0% <ø> (ø)
...lml/tests/integration_tests/test_nullable_types.py	100.0% <ø> (ø)
... and 27 more

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[bchen1116]

LGTM! I like that we're changing the name to match ww and make the terms flow with each other

[ParthivNaresh]

LGTM!