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 app. We disabled this app 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

cc @aude @manojnair