Replace all uses of deprecated function sklearn.datasets.load_boston (#7373)

* Replace all uses of deprecated function sklearn.datasets.load_boston

* More renaming

* Fix bad name

* Update assertion

* Fix n boosted rounds.

* Avoid over regularization.

* Rebase.

* Avoid over regularization.

* Whac-a-mole

Co-authored-by: fis <jm.yuan@outlook.com>

demo/guide-python/update_process.py

@@ -8,14 +8,14 @@ experiment.
 """
 
 import xgboost as xgb
-from sklearn.datasets import load_boston
+from sklearn.datasets import fetch_california_housing
 import numpy as np
 
 
 def main():
     n_rounds = 32
 
-    X, y = load_boston(return_X_y=True)
+    X, y = fetch_california_housing(return_X_y=True)
 
     # Train a model first
     X_train = X[: X.shape[0] // 2]