fix: downgrade minSdkVersion (#642)

ui/flutter/android/app/build.gradle

@@ -47,7 +47,7 @@ android {
         applicationId "com.gopeed"
         // You can update the following values to match your application needs.
         // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-build-configuration.
-        minSdkVersion 32
+        minSdkVersion 21
         targetSdkVersion 29
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName