Prepare v0.15.0 releases (#1238)

Specifically:

* [core] v0.14.12
* [fwfh_just_audio] v0.14.3
* [fwfh_webview] v0.15.0
* [enhanced] v0.15.0

Also:

* Use macOS 12 for iOS integration test (somewhat related to #1236)

Author: daohoangson

.github/workflows/flutter.yml

@@ -101,7 +101,7 @@ jobs:
 
   integration_test_ios:
     name: Integration test (iOS)
-    runs-on: macos-13
+    runs-on: macos-12
     steps:
       - uses: actions/checkout@0ad4b8fadaa221de15dcec353f45205ec38ea70b # v4.1.4
       - name: Setup Flutter

demo_app/android/app/build.gradle

@@ -28,7 +28,7 @@ android {
 
     // TODO: remove hard-coded value when `flutter.ndkVersion` is sufficient
     // open $(dirname $(dirname $(which flutter)))/packages/flutter_tools
-    ndkVersion "25.1.8937393"
+    ndkVersion "26.1.10909125"
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8