Native quantile regression for xgb 2.0.0 and above #4431

Job	Run time
docs	12m 27s
lint	1m 19s
tests (macos-latest, 3.9, all)	41m 12s
check-examples (03-FFT-examples.ipynb)	6m 0s
tests (ubuntu-latest, 3.9, all)	36m 43s
check-examples (04-RNN-examples.ipynb)	10m 41s
check-examples (00-quickstart.ipynb)	8m 27s
check-examples (02-data-processing.ipynb)	10m 7s
check-examples (01-multi-time-series-and-covariates.ipynb)	9m 43s
	2h 16m 39s

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