Allow to set non-Discover default page for app management

⚠️ This issue respects the following points: ⚠️ • This is a bug, not a question or a configuration/webserver/proxy issue. • This issue is not already reported on [Github]( OR [Nextcloud Community Forum]( _(I've searched it)_. • Nextcloud Server is up to date. See [Maintenance and Release Schedule]( for supported versions. • I agree to follow Nextcloud's [Code of Conduct]( Bug description Opening "Apps" now lands on a new "Discover" page added in v29: It means that in almost all cases, one needs to manually select an actual apps page to see available updates, installed apps or do upgrades. If there is an option to hide this panel, I couldn't find it. Steps to reproduce 1. Click 'Apps' on the use menu in the upper right corner. 2. "Discover" advertising page appears. Expected behavior Given a permanent setting, clicking 'Apps' goes to the apps installed (as in v28),…