Update iOS native library to support Apple Privacy Manifest

## Summary
Request to update the iOS native library version to `8.18.0` or later to incorporate support for Apple Privacy Manifest.

## Motivations
Staring on May 1 to upload an application to the AppStore, the SDKs must have a Privacy Manifest included that describes the use of data that the SDK makes. In the native iOS version `8.18.0` of [Amplitude](https://github.com/amplitude/Amplitude-iOS/releases) included this feature. It would be nice if you could update this dependency in the Unity SDK and verify its compatibility.

Apple Privacy Manifest links:
- https://developer.apple.com/news/?id=av1nevon
- https://developer.apple.com/news/?id=3d8a9yyh
- https://developer.apple.com/support/third-party-SDK-requirements/