Remove references to Google user migration app
## The problem
**Describe precisely the problem you're facing and add screenshots to illustrate**
We have some feedback from beta tester from murena.io https://gitlab.e.foundation/e/infra/ecloud/beta-feedback. They complain mostly about [User migration](https://doc.e.foundation/support-topics/migrate-data-from-google-cloud) app. We disabled this [app](https://gitlab.e.foundation/groups/e/-/issues/?sort=created_date&state=opened&label_name%5B%5D=Google+migration&first_page_size=100&show=eyJpaWQiOiIzMjEzIiwiZnVsbF9wYXRoIjoiZS9pbmZyYS9iYWNrbG9nIiwiaWQiOjMzMjY1fQ%3D%3D) but the doc and experimental page mention it.
**Steps to reproduce**
So we have to remove these 2 references:
* https://murena.io/settings/user/beta-user
* https://doc.e.foundation/support-topics/migrate-data-from-google-cloud
## Technical details
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cc @aude @manojnair
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