Example cleanup & improve gradle compatibility.

[timbotimbo]

Description

This fixes some errors that you might get when running the current example project, mainly for Android.

Fixes

Android Compatibility:

• Update compile and target sdk to 33.
• Update the kotlin (1.7.10), gradle (7.5) and gradle plugin (7.3.0) versions to those used by Flutter 3.13.9 projects.
• Replace deprecated jcenter() with mavencentral().

Cleanup (all platforms):

• Remove broken 'Native activity' menu option
• Remove folders for the macos and linux build targets, which are incompatible anyway.

Fixed errors

One or more plugins require a higher Android SDK version.
Fix this issue by adding the following to \example\android\app\build.gradle:
android {
  compileSdkVersion 33
  ...
}

┌─ Flutter Fix ───────────────────────────────────────────────────────────────────────────────────┐
│ [!] Your project's Gradle version is incompatible with the Java version that Flutter is using   │
│ for Gradle.       


According to the [flutter gradle migration guide](https://docs.flutter.dev/release/breaking-changes/android-java-gradle-migration-guide)
Gradle should be updated to 7.3-7.6.1

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore