CocoaPods: Set IPHONEOS_DEPLOYMENT_TARGET, and bump it to 12.2

By default, Rust targets iOS 7, which is unnecessary but fine for our
purposes. However, BoringSSL has some assembly files that were getting
compiled for the "latest" version (based on the SDK). Address both of
these by setting IPHONEOS_DEPLOYMENT_TARGET explicitly when building
as a CocoaPod.

While here, bump the CocoaPod minimum iOS version to 12.2, which
matches the main Signal app and will allow SignalCoreKit to do the
same.

LibSignalClient.podspec

@@ -14,7 +14,10 @@ Pod::Spec.new do |s|
   s.source           = { :git => 'https://github.com/signalapp/libsignal.git', :tag => "v#{s.version}" }
 
   s.swift_version    = '5'
-  s.platform         = :ios, '10'
+  # We use this to set IPHONEOS_DEPLOYMENT_TARGET below.
+  # The Rust compiler driver expects this to always be in the form 'major.minor'.
+  min_deployment_target = '12.2'
+  s.platform         = :ios, min_deployment_target
 
   s.dependency 'SignalCoreKit'
 
@@ -67,8 +70,9 @@ Pod::Spec.new do |s|
     }
   ]
 
-  s.prepare_command = %q(
+  s.prepare_command = %Q(
     set -euo pipefail
+    export IPHONEOS_DEPLOYMENT_TARGET=#{min_deployment_target}
     CARGO_BUILD_TARGET=aarch64-apple-ios swift/build_ffi.sh --release
     CARGO_BUILD_TARGET=x86_64-apple-ios swift/build_ffi.sh --release
     CARGO_BUILD_TARGET=aarch64-apple-ios-sim swift/build_ffi.sh --release