Align WebRTC-SDK version with flutter_webrtc 1.3.1 (137.7151.04 → 137.7151.12)

[antonferra]

This is the same class of issue as #856 and #875.

[!] CocoaPods could not find compatible versions for pod "WebRTC-SDK":
 In Podfile:
   flutter_webrtc was resolved to 1.2.0, which depends on
   WebRTC-SDK (= 137.7151.12)

   livekit_client was resolved to 2.6.4, which depends on
   WebRTC-SDK (= 137.7151.04)

livekit_client declares flutter_webrtc as a dependency in its podspec, so both packages are always installed together but they disagree on the exact WebRTC-SDK version.

Proposed fix: Bump WebRTC-SDK to 137.7151.12 in both ios/livekit_client.podspec and macos/livekit_client.podspec. The iOS deployment target should also move to 16.0 since WebRTC-SDK 137.7151.12 requires it.