chore: use Narwhals stable api

[MarcoGorelli]

Description

Narwhals' new release comes with a stable API: https://narwhals-dev.github.io/narwhals/backcompat/. I'd suggest using this to guard against potential changes

Fixes #(issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines (ruff)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (also to the readme.md)
• I have added tests that prove my fix is effective or that my feature works
• I have added tests to check whether the new feature adheres to the sklearn convention
• New and existing unit tests pass locally with my changes

[FBruzzesi]

Thanks Marco 🙌🏼