Set ios_deployment_target to iOS 12.0 (#808)

Build the engine Flutter.framework against a minimum of iOS 12.
Part of https://github.com/flutter/flutter/issues/140474

Flutter tooling already enforcing iOS 12 on app-end: https://github.com/flutter/flutter/pull/140478

See also https://github.com/flutter/buildroot/pull/574
diff --git a/build/config/ios/ios_sdk.gni b/build/config/ios/ios_sdk.gni
index fa9a903..06b91a5 100644
--- a/build/config/ios/ios_sdk.gni
+++ b/build/config/ios/ios_sdk.gni
@@ -16,7 +16,7 @@
   use_ios_simulator = true
 
   # Version of iOS that we're targeting.
-  ios_deployment_target = "11.0"
+  ios_deployment_target = "12.0"
 
   # The path to the iOS device SDK.
   ios_device_sdk_path = ""