Bump AGP to 8.7.0; pin builtInKotlin=false for Flutter 3.44.x

[ubxty]

Summary

• Bumps the Android Gradle Plugin pinned in android/build.gradle's
  buildscript classpath from 7.3.0 to 8.7.0, matching the direction
  the wider Flutter plugin community is moving and lining up with the
  AGP versions the example app's settings.gradle.kts already declares.
• Bumps the plugin's Kotlin Gradle Plugin classpath from 1.7.10 to
  2.0.21 so it matches the KGP 2.x toolchain the example app's
  settings.gradle.kts declares.
• Pins android.builtInKotlin=false and android.newDsl=false in the
  example's gradle.properties. On Flutter 3.44.x the Flutter Gradle
  plugin's detectApplyingKotlinGradlePlugin still calls
  pluginManager.apply("kotlin-android") on plugin subprojects during
  configuration, and AGP 9.0+ rejects the legacy
  apply plugin: "kotlin-android" when builtInKotlin=true. Keeping
  the legacy apply path working is the recommended temporary bypass
  until the project's minimum Flutter version moves past 3.44.
• Switches the example app's jvmTarget from JavaVersion.VERSION_11
  to the string "11", which the KGP 2.x kotlinOptions extension
  requires under the Kotlin DSL.
• CHANGELOG entry under 5.1.1 explaining the rationale.

Why AGP 8.7.0 (and not 7.3.0 / 9.x)?

The apply plugin: 'com.android.library' line in the plugin's
android/build.gradle is resolved against the buildscript classpath
in that file, not the AGP version the consuming app declares. Keeping
that classpath pinned at 7.3.0 is a long-standing source of
NoSuchMethodError / com.android.Version.ANDROID_GRADLE_PLUGIN_VERSION
NPEs on AGP 8+ consumer projects. Bumping to 8.7.0 keeps the plugin
aligned with the AGP 8.7+ versions that modern Flutter apps (including
this example) already pin in their settings.gradle.kts.

Verification

$ export PATH="$HOME/fvm/versions/stable/bin:$HOME/local/bin:$PATH"
$ cd example && flutter build apk --debug
✓ Built build/app/outputs/flutter-apk/app-debug.apk

on Flutter 3.44.1, Dart 3.12.1, Gradle 9.4.1, AGP 9.2.0 (the
combination the example app's settings.gradle.kts pins).

Not done in this PR (intentional)

• The example app's gradle.properties does not flip
  android.builtInKotlin to true. The full built-in Kotlin migration
  has to wait for the project's minimum Flutter version to move past
  3.44 (the Flutter 3.45+ Flutter Gradle plugin teaches
  detectApplyingKotlinGradlePlugin to skip plugin subprojects that
  set builtInKotlin=true themselves).
• The plugin's apply plugin: 'kotlin-android' line and kotlinOptions
  block remain in place for the same reason. They will be removed in
  the follow-up PR that does the full migration.