Automatic clean-up of the outdated ExApp Docker images

Describe why it is important and where it will be useful

Docker images of former versions of the ex-apps fill up the disk space.

Describe your proposed solution

1. Implement a background task for cleaning images with a configurable time interval(measurement in days, value 0 means job is disabled, default value=7).

2. Optional checkbox "clean images right after update", disabled by default.

Note: Options should be located here: http://nextcloud.local/index.php/settings/admin/app_api

Relevant API endpoint would be: https://docs.docker.com/reference/api/engine/version/v1.47/#tag/Image/operation/ImagePrune

Describe alternatives you've considered, if relevant

No response

Additional context

Additional related changes to be done in other repos in a separate PRs:

• Allow images/prune path in the haproxy config in https://github.com/nextcloud/docker-socket-proxy
• Add a high level API and allow the prune path in https://github.com/nextcloud/HaRP

Start at the Nextcloud admin AppAPI settings page and review the Docker Engine v1.47 ImagePrune endpoint. Define how the configurable interval and optional post-update cleanup should be represented and triggered, including the disabled value of 0. Done means outdated ExApp images can be cleaned on the configured schedule and optionally after updates, with both options exposed at the specified settings location.