Bump cdav-library from `76cab48` to `18b3877`

Bumps [cdav-library](https://github.com/nextcloud/cdav-library) from `76cab48` to `18b3877`.
- [Release notes](https://github.com/nextcloud/cdav-library/releases)
- [Commits](https://github.com/nextcloud/cdav-library/compare/76cab484266825b67ea0491da5200f1fe391a9a4...18b38778637386424dff1acf7f9a41b4ca757721

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