Bump go_router from 14.8.1 to 15.0.0 in /testing_app (#2611)

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Bumps [go_router](https://github.com/flutter/packages/tree/main/packages) from 14.8.1 to 15.0.0.
<details>
<summary>Commits</summary>
<ul>
<li><a href="b03228fba3"><code>b03228f</code></a> [go_router] Adds <code>caseSensitive</code> to <code>GoRoute</code> (<a href="https://github.com/flutter/packages/tree/main/packages/issues/8992">#8992</a>)</li>
<li><a href="2fcc4032dd"><code>2fcc403</code></a> [camera_android_camerax] Updates internal API wrapper to use ProxyApis (<a href="https://github.com/flutter/packages/tree/main/packages/issues/8618">#8618</a>)</li>
<li><a href="43890676d0"><code>4389067</code></a> [webview_flutter_android] Adds internal wrapper methods for native `WebViewCl...</li>
<li><a href="7bdffbe33f"><code>7bdffbe</code></a> [webview_flutter_wkwebview] Fixes <code>loadFlutterAsset</code> exception and updates na...</li>
<li><a href="f26b681ab4"><code>f26b681</code></a> [various] Update Android example toolchains (<a href="https://github.com/flutter/packages/tree/main/packages/issues/9062">#9062</a>)</li>
<li><a href="312f23f9bf"><code>312f23f</code></a> [shared_preferences] Allow reading int as long in SharedPreferences <a href="https://github.com/flutter/packages/tree/main/packages/issues/165781">#165781</a> (...</li>
<li><a href="2405f6a2b7"><code>2405f6a</code></a> [pigeon] swift equality methods (<a href="https://github.com/flutter/packages/tree/main/packages/issues/8971">#8971</a>)</li>
<li><a href="4a36dc63f7"><code>4a36dc6</code></a> [all_packages] Enforce property assignment for compile sdk over method assign...</li>
<li><a href="eaf475cefb"><code>eaf475c</code></a> [google_maps_flutter] Fix iOS info window regression (<a href="https://github.com/flutter/packages/tree/main/packages/issues/8939">#8939</a>)</li>
<li><a href="07496eb030"><code>07496eb</code></a> [multicast_dns] MDnsClient::listen supports onError callback (<a href="https://github.com/flutter/packages/tree/main/packages/issues/8888">#8888</a>)</li>
<li>Additional commits viewable in <a href="https://github.com/flutter/packages/commits/go_router-v15.0.0/packages">compare view</a></li>
</ul>
</details>
<br />

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</details>

testing_app/pubspec.yaml

@@ -12,7 +12,7 @@ dependencies:
 
   cupertino_icons: ^1.0.3
   provider: ^6.0.2
-  go_router: ">=10.0.0 <15.0.0"
+  go_router: ">=10.0.0 <16.0.0"
 
 dev_dependencies:
   analysis_defaults: