Update macOS podspec version requirement (flutter#9077)

The framework is currently built with a 10.12 deployment target, so the
podspec can be at 10.12 rather than 10.13.

The TODO is left because this is an update to the current reality rather
than a final decision about what should be supported.

Author: stuartmorgan-g

shell/platform/darwin/macos/framework/FlutterMacOS.podspec

@@ -14,6 +14,6 @@ Flutter is Google's portable UI toolkit for building beautiful, natively-compile
   s.author           = { 'Flutter Dev Team' => '[email protected]' }
   s.source           = { :git => 'https://github.com/flutter/engine', :tag => s.version.to_s }
   # TODO: Decide what value should be here. See #33200.
-  s.osx.deployment_target = '10.13'
+  s.osx.deployment_target = '10.12'
   s.vendored_frameworks = 'FlutterMacOS.framework'
 end