Bump pigeon in /add_to_app/books/flutter_module_books

Bumps [pigeon](https://github.com/flutter/packages/tree/main/packages) from 19.0.2 to 20.0.0.
- [Release notes](https://github.com/flutter/packages/releases)
- [Commits](https://github.com/flutter/packages/commits/pigeon-v20.0.0/packages)

---
updated-dependencies:
- dependency-name: pigeon
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

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 <20.0.0"
+  pigeon: ">=11.0.0 <21.0.0"
   flutter_test:
     sdk: flutter