Bump pigeon from 20.0.2 to 21.0.0 in /add_to_app/books/flutter_module_books (#2355)

Bumps [pigeon](https://github.com/flutter/packages/tree/main/packages)
from 20.0.2 to 21.0.0.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="3379e51a04"><code>3379e51</code></a>
[pigeon] Fix C++ enum naming (<a
href="https://github.com/flutter/packages/tree/main/packages/issues/7094">#7094</a>)</li>
<li><a
href="33caf1d3d5"><code>33caf1d</code></a>
[ios]Fix compile error when conforming UIApplication to Launcher due to
MainA...</li>
<li><a
href="21e3340a95"><code>21e3340</code></a>
[go_router] Makes GoRouterState lookup more robust. (<a
href="https://github.com/flutter/packages/tree/main/packages/issues/6920">#6920</a>)</li>
<li><a
href="da04812172"><code>da04812</code></a>
[ci][web] Run tests in canvaskit mode. (<a
href="https://github.com/flutter/packages/tree/main/packages/issues/6879">#6879</a>)</li>
<li><a
href="0ae6dda5d1"><code>0ae6dda</code></a>
[image_picker]: Bump androidx.core:core from 1.10.1 to 1.13.1 in
/packages/im...</li>
<li><a
href="ea35fc6fb8"><code>ea35fc6</code></a>
[camera_avfoundation] Adds Swift Package Manager compatibility (<a
href="https://github.com/flutter/packages/tree/main/packages/issues/7080">#7080</a>)</li>
<li><a
href="57d42e447c"><code>57d42e4</code></a>
[webview_flutter_wkwebview] Adds Swift Package Manager compatibility (<a
href="https://github.com/flutter/packages/tree/main/packages/issues/7091">#7091</a>)</li>
<li><a
href="c47614c6d9"><code>c47614c</code></a>
[webview_flutter_web] Migrate to package:web. (<a
href="https://github.com/flutter/packages/tree/main/packages/issues/6792">#6792</a>)</li>
<li><a
href="17188b758b"><code>17188b7</code></a>
[camera] Clean up <code>maxDuration</code> code (<a
href="https://github.com/flutter/packages/tree/main/packages/issues/7039">#7039</a>)</li>
<li><a
href="007ec6676f"><code>007ec66</code></a>
Update espresso dependencies (<a
href="https://github.com/flutter/packages/tree/main/packages/issues/7048">#7048</a>)</li>
<li>Additional commits viewable in <a
href="https://github.com/flutter/packages/commits/pigeon-v21.0.0/packages">compare
view</a></li>
</ul>
</details>
<br />


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add_to_app/books/flutter_module_books/pubspec.yaml

@@ -15,7 +15,7 @@ dependencies:
 dev_dependencies:
   analysis_defaults:
     path: ../../../analysis_defaults
-  pigeon: ">=11.0.0 <21.0.0"
+  pigeon: ">=11.0.0 <22.0.0"
   flutter_test:
     sdk: flutter