Config setting `config_is_read_only` should prevent Nextcloud from writing to `config.php`

[mejo-]

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Description

The config setting config_is_read_only currently only prevents Nextcloud from checking for writable config in server health checks.

That's also how the setting is described at https://github.com/nextcloud/documentation/blob/6efc785734a23786bff4c6340a03fa2acbf12f42/admin_manual/configuration_server/config_sample_php_parameters.rst#nextcloud-verifications.

Still, users/admins expect the setting to prevent Nextcloud from writing to config.php. That would be helpful, e.g. to prevent admin users from accidently changing server configuration.

Steps to reproduce

1. Set 'config_is_read_only' => true in config.php
2. Make changes to Nextcloud config via admin interface or occ
3. Nextcloud writes to config.php