Native quantile regression for xgb 2.0.0 and above #4434

Job	Run time
docs	8m 29s
lint	48s
tests (macos-latest, 3.9, all)	47m 17s
check-examples (03-FFT-examples.ipynb)	3m 39s
tests (ubuntu-latest, 3.9, all)	30m 49s
check-examples (04-RNN-examples.ipynb)	8m 6s
check-examples (00-quickstart.ipynb)	6m 39s
check-examples (02-data-processing.ipynb)	6m 34s
check-examples (01-multi-time-series-and-covariates.ipynb)	7m 20s
	1h 59m 41s

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