Update Android version to 20+ & iOS to 6.0.2

Author: CodeMasterYi

GoogleSignInPlugin/Assets/GoogleSignIn/Editor/GoogleSignInDependencies.xml

@@ -3,7 +3,7 @@
   <!-- See https://github.com/googlesamples/unity-jar-resolver#usage for
     how to configure dependencies -->
   <androidPackages>
-    <androidPackage spec="com.google.android.gms:play-services-auth:16+">
+    <androidPackage spec="com.google.android.gms:play-services-auth:20+">
       <androidSdkPackageIds>
         <androidSdkPackageId>extra-google-m2repository</androidSdkPackageId>
       </androidSdkPackageIds>
@@ -12,7 +12,7 @@
 
   <!-- iOS Cocoapod dependencies can be specified by each iosPod element. -->
   <iosPods>
-    <iosPod name="GoogleSignIn" version=">= 4.0.2" bitcodeEnabled="false"
+    <iosPod name="GoogleSignIn" version=">= 6.0.2" bitcodeEnabled="false"
         minTargetSdk="6.0">
     </iosPod>
   </iosPods>