build(deps): bump sumup in /backend/rust in the sumup-sdk group

Bumps the sumup-sdk group in /backend/rust with 1 update: [sumup](https://github.com/sumup/sumup-rs).


Updates `sumup` from 0.5.5 to 0.5.6
- [Release notes](https://github.com/sumup/sumup-rs/releases)
- [Changelog](https://github.com/sumup/sumup-rs/blob/main/release-plz.toml)
- [Commits](sumup/sumup-rs@v0.5.5...v0.5.6)

---
updated-dependencies:
- dependency-name: sumup
  dependency-version: 0.5.6
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: sumup-sdk
...

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Author: dependabot[bot]