Native quantile regression for xgb 2.0.0 and above #4433

Job	Run time
tests (macos-latest, 3.9, all)	47m 13s
docs	10m 39s
lint	1m 6s
check-examples (03-FFT-examples.ipynb)	3m 34s
tests (ubuntu-latest, 3.9, all)	35m 53s
check-examples (04-RNN-examples.ipynb)	14m 42s
check-examples (00-quickstart.ipynb)	11m 21s
check-examples (02-data-processing.ipynb)	8m 46s
check-examples (01-multi-time-series-and-covariates.ipynb)	7m 5s
	2h 20m 19s

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