Bump cdav-library from `76b64e6` to `c18f08c`

Bumps [cdav-library](https://github.com/nextcloud/cdav-library) from `76b64e6` to `c18f08c`.
- [Release notes](https://github.com/nextcloud/cdav-library/releases)
- [Commits](https://github.com/nextcloud/cdav-library/compare/76b64e66e8e853c1176b35d12236202604147799...c18f08c2273216a196a62a2318956a1820e942f3

)

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