Change default objective to MedianAE for time series regression

[ParthivNaresh]

Fixes #3204

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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 #3205 (4338709) into main (8553afc) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3205     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        326     326             
  Lines      31135   31137      +2     
=======================================
+ Hits       31039   31041      +2     
  Misses        96      96             

Impacted Files	Coverage Δ
evalml/automl/utils.py	100.0% <ø> (ø)
evalml/tests/automl_tests/test_automl_utils.py	100.0% <100.0%> (ø)

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[jeremyliweishih]

Thank you!