Bump pypdf from 5.9.0 to 6.0.0 in /backend (#666)

Bumps [pypdf](https://github.com/py-pdf/pypdf) from 5.9.0 to 6.0.0.
- [Release notes](https://github.com/py-pdf/pypdf/releases)
- [Changelog](https://github.com/py-pdf/pypdf/blob/main/CHANGELOG.md)
- [Commits](py-pdf/pypdf@5.9.0...6.0.0)

---
updated-dependencies:
- dependency-name: pypdf
  dependency-version: 6.0.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: Cyrill Püntener <34008738+wp99cp@users.noreply.github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -8,4 +8,4 @@ openpyxl==3.1.5
 grequests==0.7.0
 pyclustering==0.10.1.2
 polyline==2.0.3
-pypdf==5.9.0
+pypdf==6.0.0