Fix: removing "useProguard true" flag

Removing the flag since it has become obsolete
Supporting documents
https://stackoverflow.com/questions/52428973/how-to-minify-a-flutter-app-with-proguard -> https://stackoverflow.com/questions/57635963/gradle-dsl-element-useproguard-is-obsolete-and-will-be-removed-soon

README.md

@@ -131,9 +131,8 @@ buildTypes {
         // Signing with the debug keys for now, so `flutter run --release` works.
         signingConfig signingConfigs.debug
         
-        // Add below 3 lines for proguard
+        // Add below 2 lines for proguard
         minifyEnabled true
-        useProguard true
         proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
     }
 }