Categorize `map_flatten` as a style lint instead of pedantic

[orhun]

I recently learned that we can write and_then() for Option's instead of map().flatten(). There is a lint for that which is called map_flatten. Unfortunately, it is categorized as clippy::pedantic so I doubt I would ever hear about it if I didn't come to this repository to propose the exact same lint and see that it already exists. I think this should be suggested by clippy as default.

So I'm basically proposing to make map_flatten a style lint instead of pedantic so that clippy will warn about this lint as default. I'd like to provide a PR.

Additional context

• Suggest replacing .map(f).flatten() with .and_then(f) on Options #6870 (comment)

@rustbot label C-enhancement