Ubuntu ppa 26.10 dev: signing key seems to be outdated

### ⚠️ Before submitting, please verify the following: ⚠️

- [x] This is a **bug**, not a question or a configuration issue.
- [x] This issue is **not** already reported on Github (I have searched for it).
- [x] Nextcloud Server and Desktop Client are **up to date**. See [Server Maintenance and Release Schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule) and [Desktop Releases](https://nextcloud.com/install/#install-clients) for supported versions.
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/)

### Bug description

Using the Ubuntu Stonking (dev) ppa fails due to the ppa signing key not adhering to the latest Ubuntu security restrictions.

### Steps to reproduce

1. Update the apt .sources file
2. run apt update
3. Error

### Expected behavior

Update to the new Stonking version provided on Launchpad fails.

Fehler: Das Depot »https://ppa.launchpadcontent.net/nextcloud-devs/client/ubuntu stonking InRelease« ist nicht signiert.
Hinweis: Eine Aktualisierung von solch einem Depot kann nicht auf eine sichere Art durchgeführt werden, daher ist es standardmäßig deaktiviert.
Hinweis: Weitere Details zur Erzeugung von Paketdepots sowie zu deren Benutzerkonfiguration finden Sie in der Handbuchseite apt-secure(8).
Warnung: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. Signature verification failed: http://archive.ubuntu.com/ubuntu stonking InRelease: Sub-process /usr/bin/sqv returned an error code (1), error message is: Error: Invalid value for SEQUOIA_CRYPTO_POLICY Caused by: Relative path not allowed: standard

### Which files are affected by this bug

/etc/apt/keyrings/nextcloud-ppa.asc

### Operating system

Linux

### Which version of the operating system you are running.

Ubuntu 26.10 dev (Stonking)

### Installation method

Community PPA (please test with the AppImage package ?)

### Nextcloud Server version

34.0.4

### Nextcloud Desktop Client version

34.0.3-20260826.113916.7563f574a8-1.0~stonking1

### Did this occur after an update or on a clean installation?

Minor version update (i.e. 33.0.0 → 33.0.1)

### Are you using the Nextcloud Server Encryption module?

No

### Are you using an external user-backend?

- [x] Default internal user-backend
- [ ] LDAP or Active Directory
- [ ] SSO - SAML
- [ ] Other

### Nextcloud Server logs

```shell

```

### Additional info

_No response_

Start by inspecting /etc/apt/keyrings/nextcloud-ppa.asc and reproducing the failure with apt update on Ubuntu 26.10 dev. Check how the Community PPA signing key is published and consumed; done when the Stonking repository passes signature verification and updates normally.

Written by the indexing model from the issue text.