Update Nextcloud stuck to 15.0.14

[Maniack_Crudelis]

The version 18.0.1 is currently into testing period.

[Nextcloud] A cloud for personal or group use

New testing release This release really need to be tested, please give us feedback 14 March 2020 - #260 Upgrade to nextcloud 18.0.1 Add some php dependencies as suggested in https://docs.nextcloud.com/server/18/admin_manual/installation/source_installation.html to make some features working Use php7.3 Fix php issues with php7.4 sudo yunohost app install https://github.com/YunoHost-Apps/nextcloud_ynh/tree/testing --debug or sudo yunohost app upgrade nextcloud -u https://github.com/…

The git repository is set to the testing branch.
So indeed, your app is up to date.