flet_example/android: bump KGP 2.1.0 → 2.3.21 (match screen_brightness_android stdlib)

screen_brightness_android 2.1.5 ships compiled against Kotlin 2.3.21
stdlib. KGP 2.1 couldn't read the metadata:

  Module was compiled with an incompatible version of Kotlin.
  The binary version of its metadata is 2.3.0, expected version is 2.1.0.

Bumping KGP to 2.3.21 matches the transitive stdlib version.

Author: FeodorFitsner

src/serious_python/example/flet_example/android/settings.gradle

@@ -21,9 +21,10 @@ plugins {
     // AGP 8.9.1+ required by androidx.core 1.18.0 (transitive via Flet);
     // AGP 8.9 requires Gradle 8.11+ (see gradle-wrapper.properties).
     id "com.android.application" version "8.9.1" apply false
-    // KGP 2.0+ required by screen_brightness_android 2.1.5's use of the
-    // `kotlin.compilerOptions { ... }` DSL.
-    id "org.jetbrains.kotlin.android" version "2.1.0" apply false
+    // KGP 2.3.x required: screen_brightness_android 2.1.5 ships with a
+    // Kotlin 2.3.21 stdlib whose metadata format (binary version 2.3.0)
+    // can't be read by KGP < 2.3.
+    id "org.jetbrains.kotlin.android" version "2.3.21" apply false
 }
 
 include ":app"