Change default n_jobs value for XGBoost

[bchen1116]

fix #2437

Original Perf test results here

[codecov]

Codecov Report

Merging #2757 (c52402c) into main (84df8af) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #2757   +/-   ##
=====================================
  Coverage   99.9%   99.9%           
=====================================
  Files        300     300           
  Lines      27705   27705           
=====================================
  Hits       27656   27656           
  Misses        49      49           

Impacted Files	Coverage Δ
...nents/estimators/classifiers/xgboost_classifier.py	100.0% <ø> (ø)
...ponents/estimators/regressors/xgboost_regressor.py	100.0% <ø> (ø)
evalml/tests/component_tests/test_components.py	100.0% <ø> (ø)

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[jeremyliweishih]

good work!