config: upgrade gradle 8.14.3, agp 8.11.1, kotlin 2.2.20

Flutter 3.47 menolak build dengan Gradle < 8.14.0, AGP < 8.11.1, dan
Kotlin < 2.2.20.

Hapus juga assets/icons/ dan assets/json/ dari pubspec: foldernya tidak
pernah ada dan tidak dirujuk dari lib/, menyebabkan error "unable to find
directory entry" saat build.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

android/gradle/wrapper/gradle-wrapper.properties

@@ -2,4 +2,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.12-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.14.3-all.zip

android/settings.gradle.kts

@@ -18,8 +18,8 @@ pluginManagement {
 
 plugins {
     id("dev.flutter.flutter-plugin-loader") version "1.0.0"
-    id("com.android.application") version "8.7.3" apply false
-    id("org.jetbrains.kotlin.android") version "2.1.0" apply false
+    id("com.android.application") version "8.11.1" apply false
+    id("org.jetbrains.kotlin.android") version "2.2.20" apply false
     id("com.google.gms.google-services") version "4.4.4" apply false
 }