Update minimum iOS support to 12 (#445)

GoogleSignIn.podspec

@@ -13,7 +13,7 @@ The Google Sign-In SDK allows users to sign in with their Google account from th
     :tag => s.version.to_s
   }
   s.swift_version = '4.0'
-  ios_deployment_target = '11.0'
+  ios_deployment_target = '12.0'
   osx_deployment_target = '10.15'
   s.ios.deployment_target = ios_deployment_target
   s.osx.deployment_target = osx_deployment_target

Package.swift

@@ -24,7 +24,7 @@ let package = Package(
   defaultLocalization: "en",
   platforms: [
     .macOS(.v10_15),
-    .iOS(.v11)
+    .iOS(.v12)
   ],
   products: [
     .library(