Rename to iOS SDK

I believe we should rename this from the `swift-sdk` to the `ios-sdk`. The reality is that this SDK only currently works on iOS, and its main intended use is for Apple's platforms (we should be able to add support for `tvOS` / `visionOS` / `watchOS` / `macOS` fairly easily). It also seems that most companies within the industry still use `ios-sdk` for a Swift SDK that supports other parts of the Apple ecosystem.

From a marketing perspective, iOS developers coming to OpenFeature are searching for an iOS Client SDK, we also already call this SDK the `OpenFeature iOS SDK` in our public documentation: https://openfeature.dev/docs/reference/technologies/client/swift

I am happy to help drive the changes to the repo name, documentation, and whatever else would be required. But I believe that because we are only using SPM, all that would be required is:
- Rename repo from `swift-sdk` to `ios-sdk`
- Update repo `README.md` and any other repo settings from `swift-sdk` to `ios-sdk`
- Update public openfeature.dev documentation