Update Kotlin and AGP to meet current Flutter requirements

- Update Kotlin from 1.7.10 to 1.9.0 (Flutter minimum is 1.8.10)
- Update Android Gradle Plugin from 8.3.0 to 8.6.0 (Flutter minimum)
- Update versions in both plugin and example app

Fixes: "Your project's Kotlin version (1.7.10) is lower than Flutter's
minimum supported version of 1.8.10"

Author: claude

android/build.gradle

@@ -2,14 +2,14 @@ group 'com.nschairer.keyboard_height_plugin'
 version '1.0-SNAPSHOT'
 
 buildscript {
-    ext.kotlin_version = '1.7.10'
+    ext.kotlin_version = '1.9.0'
     repositories {
         google()
         mavenCentral()
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:8.3.0'
+        classpath 'com.android.tools.build:gradle:8.6.0'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }

example/android/settings.gradle

@@ -18,8 +18,8 @@ pluginManagement {
 
 plugins {
     id "dev.flutter.flutter-plugin-loader" version "1.0.0"
-    id "com.android.application" version "8.3.0" apply false
-    id "org.jetbrains.kotlin.android" version "1.7.10" apply false
+    id "com.android.application" version "8.6.0" apply false
+    id "org.jetbrains.kotlin.android" version "1.9.0" apply false
 }
 
 include ":app"