Update AppVeyor Node versions (#27980)

Summary:
The current AppVeyor test is failing on node v8. Since v8 is no longer active (https://nodejs.org/en/about/releases/), updated to the current LTS versions.

## Changelog

[Internal] [Changed] - Changed AppVeyor Node test versions to 10 & 12
Pull Request resolved: https://github.com/facebook/react-native/pull/27980

Test Plan: Hopefully makes the tests green �

Differential Revision: D19873610

Pulled By: hramos

fbshipit-source-id: 6f42a6046a1c015d327b15dc9537d0c2bb27e978

.appveyor/config.yml

@@ -10,8 +10,8 @@ environment:
   NDK_TOOLS_URL: https://dl.google.com/android/repository/android-ndk-r19c-windows-x86_64.zip
 
   matrix:
-    - nodejs_version: 8
     - nodejs_version: 10
+    - nodejs_version: 12
 
 install:
   # Install Android SDK Tools