Update split_data with new default for timeseries

[eccabay]

Reduces the default test_size in split_data to be 0.1 instead of 0.2, as long as the new value is greater than the passed in forecast horizon.

[codecov[bot]]

Codecov Report

Merging #3650 (874f81a) into main (994d33a) will increase coverage by 0.1%. The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3650     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        335     335             
  Lines      33790   33840     +50     
=======================================
+ Hits       33662   33712     +50     
  Misses       128     128             

Impacted Files	Coverage Δ
evalml/preprocessing/utils.py	100.0% <100.0%> (ø)
evalml/tests/conftest.py	97.9% <100.0%> (+0.1%)	:arrow_up:
...valml/tests/preprocessing_tests/test_split_data.py	100.0% <100.0%> (ø)

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