Trigger warning if `build-system.requires` contains cmake or ninja

[LecrisUT]

Since explicitly adding cmake and ninja to the build-system.requires can cause so much trouble, how about parsing the field and trigger a deprecation warning to move that to tool.scikit-build, and eventually block the build if it's still defined there?

[henryiii]

We can add the warning when we add #777 for the scikit-build-core min version. I don't think we should block, but a warning is probably fine.