android: restore compileSdk version to default

[emersion]

Flutter's default version is now higher than 31, so we don't need to manually set this anymore:
https://github.com/flutter/flutter/blob/5c0c9e9e9ad2eec7dc28b216fa55b8c5bf6d7ad9/packages/flutter_tools/gradle/src/main/kotlin/FlutterExtension.kt#L23

Fixes compilation errors with AGP v8.13:

   1.  Dependency 'androidx.fragment:fragment:1.7.1' requires libraries and applications that
       depend on it to compile against version 34 or later of the
       Android APIs.

       :webcrypto is currently compiled against android-31.

       Recommended action: Update this project to use a newer compileSdk
       of at least 34, for example 36.

       Note that updating a library or application's compileSdk (which
       allows newer APIs to be used) can be done separately from updating
       targetSdk (which opts the app in to new runtime behavior) and
       minSdk (which determines which devices the app can be installed
       on).