[sign_in_with_apple] Fix gradle warning / deprecation

https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply

```
You are applying Flutter's app_plugin_loader Gradle plugin imperatively using the apply script method, which is deprecated and will be removed in a future release. Migrate to applying Gradle plugins with the declarative plugins block: https://flutter.dev/go/flutter-gradle-plugin-apply

You are applying Flutter's main Gradle plugin imperatively using the apply script method, which is deprecated and will be removed in a future release. Migrate to applying Gradle plugins with the declarative plugins block: https://flutter.dev/go/flutter-gradle-plugin-apply
```

packages/sign_in_with_apple/sign_in_with_apple/CHANGELOG.md

@@ -1,6 +1,10 @@
+## 6.1.2
+
+- Switch to modern Gradle setup (https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply) for the example app
+
 ## 6.1.1
 
-- Removes references to Flutter v1 android embedding classes.
+- Removes references to Flutter v1 Android embedding classes.
 
 ## 6.1.0
 

packages/sign_in_with_apple/sign_in_with_apple/example/android/app/build.gradle

@@ -1,3 +1,9 @@
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -6,11 +12,6 @@ if (localPropertiesFile.exists()) {
     }
 }
 
-def flutterRoot = localProperties.getProperty('flutter.sdk')
-if (flutterRoot == null) {
-    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
-}
-
 def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
 if (flutterVersionCode == null) {
     flutterVersionCode = '1'
@@ -21,10 +22,6 @@ if (flutterVersionName == null) {
     flutterVersionName = '1.0'
 }
 
-apply plugin: 'com.android.application'
-apply plugin: 'kotlin-android'
-apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
-
 android {
     compileSdkVersion flutter.compileSdkVersion
 
@@ -62,7 +59,3 @@ android {
 flutter {
     source '../..'
 }
-
-dependencies {
-    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
-}

packages/sign_in_with_apple/sign_in_with_apple/example/android/build.gradle

@@ -1,16 +1,3 @@
-buildscript {
-    ext.kotlin_version = '1.6.0'
-    repositories {
-        google()
-        mavenCentral()
-    }
-
-    dependencies {
-        classpath 'com.android.tools.build:gradle:7.2.2'
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
-    }
-}
-
 allprojects {
     repositories {
         google()

packages/sign_in_with_apple/sign_in_with_apple/example/android/settings.gradle

@@ -1,11 +1,25 @@
-include ':app'
+pluginManagement {
+    def flutterSdkPath = {
+        def properties = new Properties()
+        file("local.properties").withInputStream { properties.load(it) }
+        def flutterSdkPath = properties.getProperty("flutter.sdk")
+        assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
+        return flutterSdkPath
+    }()
 
-def localPropertiesFile = new File(rootProject.projectDir, "local.properties")
-def properties = new Properties()
+    includeBuild("$flutterSdkPath/packages/flutter_tools/gradle")
 
-assert localPropertiesFile.exists()
-localPropertiesFile.withReader("UTF-8") { reader -> properties.load(reader) }
+    repositories {
+        google()
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}
 
-def flutterSdkPath = properties.getProperty("flutter.sdk")
-assert flutterSdkPath != null, "flutter.sdk not set in local.properties"
-apply from: "$flutterSdkPath/packages/flutter_tools/gradle/app_plugin_loader.gradle"
+plugins {
+    id "dev.flutter.flutter-plugin-loader" version "1.0.0"
+    id "com.android.application" version "7.4.2" apply false
+    id "org.jetbrains.kotlin.android" version "1.7.10" apply false
+}
+
+include ":app"

packages/sign_in_with_apple/sign_in_with_apple/pubspec.yaml

@@ -1,6 +1,6 @@
 name: sign_in_with_apple
 description: Flutter bridge to initiate Sign in with Apple (on iOS, macOS, and Android). Includes support for keychain entries as well as signing in with an Apple ID.
-version: 6.1.1
+version: 6.1.2
 homepage: https://github.com/aboutyou/dart_packages/tree/master/packages/sign_in_with_apple
 repository: https://github.com/aboutyou/dart_packages