Bump rubocop from 0.50.0 to 0.51.0 in /backend
Bump rubocop from 0.50.0 to 0.51.0 in /backend

Bumps [rubocop](https://github.com/bbatsov/rubocop) from 0.50.0 to 0.51.0.
- [Release notes](https://github.com/bbatsov/rubocop/releases/tag/v0.51.0)
- [Changelog](https://github.com/bbatsov/rubocop/blob/master/CHANGELOG.md)
- [Commits](https://github.com/bbatsov/rubocop/compare/v0.50.0...v0.51.0)
roschaefer/rundfunk-mitbestimmen,roschaefer/rundfunk-mitbestimmen,roschaefer/rundfunk-mitbestimmen