[@mantine/core] Group: use css selectors to style children

[drj17]

Replaces the logic in <Group /> that passes className as prop with a direct child selector. This removes the requirement for Group children to accept className as a prop.

[rtivital]

Thanks!

[danielemesh]

@drj17 this makes it impossible to override a Group's direct child flex-grow rule without using !important or another css selector that is more "powerful" than > * 😢
and IMHO it's a breaking change that needs to be documented...