fix: restore android example min sdk to 21

Revert the example app minSdk configuration to 21 to keep release APK builds stable with core library desugaring in CI.

Author: denischilik

example/android/app/build.gradle

@@ -49,7 +49,7 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "com.mparticle.mparticle_flutter_sdk_example"
-        minSdkVersion flutter.minSdkVersion
+        minSdkVersion 21
         targetSdkVersion 35
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName