Native quantile regression for xgb 2.0.0 and above #4432

Job	Run time
tests (macos-latest, 3.9, all)	36m 22s
docs	6m 49s
lint	1m 11s
tests (ubuntu-latest, 3.9, all)	29m 57s
check-examples (03-FFT-examples.ipynb)	5m 58s
check-examples (04-RNN-examples.ipynb)	10m 53s
check-examples (00-quickstart.ipynb)	9m 35s
check-examples (02-data-processing.ipynb)	8m 24s
check-examples (01-multi-time-series-and-covariates.ipynb)	8m 55s
	1h 58m 4s

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