Admin UI maintenance mode during Shopware updates

[renaudhager]

Improvement

We would like to have a maintenance mode during Shopware updates for the admin UI.
To do this:

• The Shopware Operatror will run an HTTP server with a maintenance page.
• When an update is being done, before running the setup/migration job, the Shopware Operator update the admin ingress to send all this traffic to its maintenance page.
• When the update is done, the Operator switches back to its original configuration, the admin ingress.

Reason

We notice that during a Shopware update, if some parts of the admin UI is accessed, it might have bad consequences (for instance plugin deletion).

Information

No response

Definition of Done

The Shopware Operator runs a web server, and update the ingress before and after a Shopware update.