Move `excessive_nesting` to `pedantic`

This was originally put in complexity but with a useless value set as the config option.

I don't really see this as being beneficial anymore as other lints, such as too_many_lines, could do the same, but are completely fine in pedantic and are probably acted upon when using #![warn(clippy::pedantic). I think we should do the same with a value like 6, which seems in the same general ballpark as too_many_lines' 100.