⬆️ Bump extern/qfr from 76dfda6 to 6b897ff

Bumps [extern/qfr](https://github.com/cda-tum/qfr) from `76dfda6` to `6b897ff`.
- [Release notes](https://github.com/cda-tum/qfr/releases)
- [Commits](cda-tum/mqt-core@76dfda6...6b897ff)

---
updated-dependencies:
- dependency-name: extern/qfr
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>