Update global learners to work with scikit-learn 1.6.0

[SvenKlaassen]

• Remove scikit-learn<1.6.0 restriction (see Sklearn 1.6.0 compatibility #290 )
• Update data checks for global estimators
• Add unit test for stacked global learners

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[OliverSchacht]

Looks good, thanks for fixing

[JanTeichertKluge]

The changes are well-structured and aim to improve the reliability and compatibility of the code.

Thanks @SvenKlaassen