MNT Fix CI issue from new scikit-learn==1.8.0

[divakaivan]

Reference Issues/PRs

Fixes #502

What does this implement/fix? Explain your changes.

the issue stems from the change here - prior to sklearn 1.8 the collinearity case in QDA was a warning, now (in 1.8) it's an error. The warnings were not seen as they are supressed by the skops test setup.

Additions:

• regularization to the QDA in tests to avoid the collinearity error
• adding a new ci-sklearn18 to the gha
• pyproject.toml update for the new version

[adrinjalali]

Otherwise LGTM. Thanks

[adrinjalali]

Catboost seems failing, they're infamously slow to release for new dependencies, you can ignore them for now for this CI with a todo ideally pointing to an existing issue (or one you'd create) on the catboost repo.

[divakaivan]

• The docs build seems to be failing because of the fetch_california_housing 403 error - https://app.readthedocs.org/api/v2/build/30885603.txt - we can use this temp solution if needed
• I commented out catboost in the 1.8 deps and added a comment to see things pass but not sure if you want it to remain like this?

[adrinjalali]

The fetch california housing issue seems to be what we have in scikit-learn too.