[Roadmap] 1.4.2 Patch release

[trivialfis]

This will be a bug fix release after 1.4.1. Please comment if you think there are commits that should be backported.
@dmlc/xgboost-committer

The release is planned to be Python only with following patches:

• CI fixes ([backport] CI fixes #6933).
• Ensure predict leaf output 1-dim vector where there's only 1 tree. #6889
• Fix for updated cupy with lazy_isinstance. ([backport] CI fixes #6933)
• Enforce input data is not object. #6927
• Fix multiclass auc with empty dataset. #6947