zulip plugin: Use flutter.compileSdkVersion for compileSdk

To match our `android/app/build.gradle`, and it is similar to a
change that happened for Flutter's first-party plugins:
  flutter/packages@bfad6b9

Author: rajveermalviya

packages/zulip_plugin/android/build.gradle

@@ -10,7 +10,7 @@ android {
     // so the settings below have no real effect.  The Gradle project exists only
     // because the Flutter Gradle plugin expects every Flutter plugin to have one.
 
-    compileSdk 34
+    compileSdk flutter.compileSdkVersion
 
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_1_8