Upgrade Nextcloud to latest v27

[binarykitchen]

Because new Nextcloud vulnerabilities have been reported and look scary, we should consider upgrading Nextcloud from the current v26 to the latest v27 and ensure it truly fixes that vulnerability.

But such a PR would depend on upgrading to PHP v8.1 first, which is covered in the other ticket (dependency): https://github.com/mail-in-a-box/mailinabox/issues/2398

Make sure all is exactly covered as precisely documented in this migration guide: docs.nextcloud.com/server/latest/admin_manual/release_notes/upgrade_to_27.html