Update gradle lockfiles template (#140115)

This PR updates almost* all Gradle buildscripts in the Flutter repo the `example` and `dev` (in particular, in `dev/integration_tests` and in `dev/benchmarks`) directories to apply Flutter's Gradle plugins using the declarative `plugins {}` block.

*almost, because:
- add-to-app (aka hybrid) apps are not migrated (related https://github.com/flutter/flutter/issues/138756)
- apps that purposefully use build files to ensure backward compatibility (e.g. [`gradle_deprecated_settings`](https://github.com/flutter/flutter/tree/3.16.0/dev/integration_tests/gradle_deprecated_settings))

dev/tools/bin/generate_gradle_lockfiles.dart

@@ -7,6 +7,7 @@
 // To regenerate these files, run `find . -type d -name 'android' | dart dev/tools/bin/generate_gradle_lockfiles.dart`
 
 import 'dart:io';
+
 import 'package:file/file.dart';
 import 'package:file/local.dart';
 import 'package:path/path.dart' as path;
@@ -139,23 +140,6 @@ const String rootGradleFileContent = r'''
 // To update all the build.gradle files in the Flutter repo,
 // See dev/tools/bin/generate_gradle_lockfiles.dart.
 
-buildscript {
-    ext.kotlin_version = '1.7.10'
-    repositories {
-        google()
-        mavenCentral()
-    }
-
-    dependencies {
-        classpath 'com.android.tools.build:gradle:7.3.0'
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
-    }
-
-    configurations.classpath {
-        resolutionStrategy.activateDependencyLocking()
-    }
-}
-
 allprojects {
     repositories {
         google()
@@ -205,12 +189,25 @@ pluginManagement {
 
     includeBuild("${settings.ext.flutterSdkPath}/packages/flutter_tools/gradle")
 
-    plugins {
-        id "dev.flutter.flutter-gradle-plugin" version "1.0.0" apply false
+    repositories {
+        google()
+        mavenCentral()
+        gradlePluginPortal()
     }
 }
 
-include ":app"
+buildscript {
+    dependencyLocking {
+        lockFile = file("${rootProject.projectDir}/buildscript-gradle.lockfile")
+        lockAllConfigurations()
+    }
+}
 
-apply from: "${settings.ext.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 "7.3.0" apply false
+    id "org.jetbrains.kotlin.android" version "1.7.10" apply false
+}
+
+include ":app"
 ''';