Bump cdav-library from `75c1b97` to `21937a6`

Bumps [cdav-library](https://github.com/nextcloud/cdav-library) from `75c1b97` to `21937a6`.
- [Release notes](https://github.com/nextcloud/cdav-library/releases)
- [Commits](https://github.com/nextcloud/cdav-library/compare/75c1b972bffa4f665ff26fd4e4f85856608ad434...21937a683f77863411039bb183f5552cce3b1471

)

---
updated-dependencies:
- dependency-name: cdav-library
  dependency-type: direct:production
...

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