Upgrade android_splash_screen Gradle configuration (#2779)

I noticed that the `android_splash_screen` sample was outdated and was
not runnable due to gradle issues.

This commit fixes the Gradle setup for the `android_splash_screen`
sample.

Key changes include:
*   Upgrading Gradle wrapper from version 6.7 to 8.7.
* Migrating to the plugins block in `settings.gradle` and
`app/build.gradle` for applying Android, Kotlin, and Flutter Gradle
plugins.
* Removing the outdated `buildscript` block from the root
`build.gradle`.
*   Updating `compileSdkVersion` and `targetSdkVersion` to 34.
*   Setting `minSdkVersion` to `flutter.minSdkVersion`.
* Adding the `namespace` property as required by newer Android Gradle
Plugin versions.
*   Refactoring Kotlin constant declarations in `MainActivity.kt`.

@ericwindmill 

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

android_splash_screen/android/app/build.gradle

@@ -1,3 +1,10 @@
+plugins {
+    id "com.android.application"
+    id "kotlin-android"
+    // The Flutter Gradle Plugin must be applied after the Android and Kotlin Gradle plugins.
+    id "dev.flutter.flutter-gradle-plugin"
+}
+
 def localProperties = new Properties()
 def localPropertiesFile = rootProject.file('local.properties')
 if (localPropertiesFile.exists()) {
@@ -21,12 +28,9 @@ 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 31
+    namespace "com.example.splash_screen_sample"
+    compileSdkVersion 34
 
     sourceSets {
         main.java.srcDirs += 'src/main/kotlin'
@@ -35,12 +39,21 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "com.example.splash_screen_sample"
-        minSdkVersion 21
-        targetSdkVersion 30
+        minSdkVersion flutter.minSdkVersion
+        targetSdkVersion 34
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName
     }
 
+    compileOptions {
+        sourceCompatibility = JavaVersion.VERSION_1_8
+        targetCompatibility = JavaVersion.VERSION_1_8
+    }
+
+    kotlinOptions {
+        jvmTarget = JavaVersion.VERSION_1_8
+    }
+
     buildTypes {
         release {
             // TODO: Add your own signing config for the release build.
@@ -55,7 +68,6 @@ flutter {
 }
 
 dependencies {
-    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
     implementation "androidx.core:core-splashscreen:1.0.0-alpha02"
     implementation "androidx.core:core:1.5.0-alpha05"
     implementation "androidx.core:core-ktx:1.6.0"