Replace `resignFirstResponder` with `@FocusState`

[pmusolino]

Reported here: #13329

In the class AddEditCoupon:

UIApplication.shared.sendAction(#selector(UIResponder.resignFirstResponder), to: nil, from: nil, for: nil)

should be replaced with

@FocusState

While addressing this issue, also check if there is similar technical debt in other SwiftUI classes.

[dangermattic]

🚫 Please add a type label (e.g. type: enhancement) and a feature label (e.g. feature: stats) to this issue.

[JorgeMucientes]

This minor refactor has been stale for a long time. Do you think we'll ever prioritise it? Can we close it this? @selanthiraiyan @itsmeichigo (please do so if needed)

[itsmeichigo]

I think the referenced PR is incorrect because this issue has nothing to do with product creation AI. Anyway, I don't think we're prioritizing coupons flow anytime soon so I'm good with closing this issue. The techdebt can be fixed when we fix any other UX issue in this flow (I think there are such issues existing already).