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I noticed that the `android_splash_screen` sample was outdated and was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* Removing the outdated `buildscript` block from the root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/build.gradle

@@ -1,16 +1,3 @@
-buildscript {
-    ext.kotlin_version = '1.5.31'
-    repositories {
-        google()
-        mavenCentral()
-    }
-
-    dependencies {
-        classpath 'com.android.tools.build:gradle:4.1.0'
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
-    }
-}
-
 allprojects {
     repositories {
         google()
@@ -18,12 +5,14 @@ allprojects {
     }
 }
 
-rootProject.buildDir = '../build'
+rootProject.buildDir = "../build"
 subprojects {
     project.buildDir = "${rootProject.buildDir}/${project.name}"
-    project.evaluationDependsOn(':app')
+}
+subprojects {
+    project.evaluationDependsOn(":app")
 }
 
-task clean(type: Delete) {
+tasks.register("clean", Delete) {
     delete rootProject.buildDir
 }