Bump active_model_serializers from 0.10.8 to 0.10.9 in /backend
Bump active_model_serializers from 0.10.8 to 0.10.9 in /backend

Bumps [active_model_serializers](https://github.com/rails-api/active_model_serializers) from 0.10.8 to 0.10.9.
- [Release notes](https://github.com/rails-api/active_model_serializers/releases)
- [Changelog](https://github.com/rails-api/active_model_serializers/blob/v0.10.9/CHANGELOG.md)
- [Commits](https://github.com/rails-api/active_model_serializers/compare/v0.10.8...v0.10.9)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
roschaefer/rundfunk-mitbestimmen,roschaefer/rundfunk-mitbestimmen,roschaefer/rundfunk-mitbestimmen