Method `setServerZone` is not fully implemented for iOS

## Expected Behavior
The iOS build via Xcode is successful

## Current Behavior
Build fails with

```
Assets/Plugins/iOS/Amplitude/AmplitudeCWrapper.m:199:60: error: no visible @interface for 'Amplitude' declares the selector 'setServerZone:updateServerUrl:'
[[Amplitude instanceWithName:ToNSString(instanceName)] setServerZone:ampServerZone updateServerUrl:updateServerUrl];
```

## Possible Solution
Either extend `Amplitude.h` and `Amplitude.m` with neccessary methods or could it be that the `AMPServerZone.h` and `.m` files are missing?

## Steps to Reproduce
1. Try to build a Unity app for iOS

## Environment
- Unity Plugin Version: 2.7.0
- Device: build step
- Device OS and Version: iOS