Update christophwurst/nextcloud requirement from ^14.0 to ^15.0
Update christophwurst/nextcloud requirement from ^14.0 to ^15.0

Updates the requirements on [christophwurst/nextcloud](https://github.com/ChristophWurst/nextcloud_composer) to permit the latest version.
- [Release notes](https://github.com/ChristophWurst/nextcloud_composer/releases)
- [Commits](https://github.com/ChristophWurst/nextcloud_composer/compare/14.0.0...v15.0.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>

juliushaertl/deck,nextcloud/deck,juliushaertl/deck,nextcloud/deck,nextcloud/deck,juliushaertl/deck