build(android): Update android example's compileSdkVersion

Reference: https://github.com/Baseflow/flutter_cache_manager/runs/5953471952

Author: sidrao2006

flutter_cache_manager/example/android/app/build.gradle

@@ -26,7 +26,7 @@ apply plugin: 'kotlin-android'
 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
-    compileSdkVersion 30
+    compileSdkVersion 31
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8