Update Kotlin to version 2.1.0

- Update Kotlin from 1.9.0 to 2.1.0 in plugin build.gradle
- Update Kotlin from 1.9.0 to 2.1.0 in example app build.gradle
- Update Kotlin from 1.9.0 to 2.1.0 in example app settings.gradle

This brings the plugin up to date with the latest stable Kotlin version.

Author: claude

android/build.gradle

@@ -2,7 +2,7 @@ group 'com.nschairer.keyboard_height_plugin'
 version '1.0-SNAPSHOT'
 
 buildscript {
-    ext.kotlin_version = '1.9.0'
+    ext.kotlin_version = '2.1.0'
     repositories {
         google()
         mavenCentral()

example/android/build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-    ext.kotlin_version = '1.9.0'
+    ext.kotlin_version = '2.1.0'
     repositories {
         google()
         mavenCentral()

example/android/settings.gradle

@@ -19,7 +19,7 @@ pluginManagement {
 plugins {
     id "dev.flutter.flutter-plugin-loader" version "1.0.0"
     id "com.android.application" version "8.6.0" apply false
-    id "org.jetbrains.kotlin.android" version "1.9.0" apply false
+    id "org.jetbrains.kotlin.android" version "2.1.0" apply false
 }
 
 include ":app"