update minSdkVersion

Author: SelaseKay

packages/firebase_snippets_app/android/app/build.gradle

@@ -50,7 +50,7 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "com.example.firebase_snippets_app"
-        minSdkVersion 21
+        minSdkVersion 23
         targetSdkVersion flutter.targetSdkVersion
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName