Fix the position of Joining a project alert

What does this MR do and why?

Resolves #472060. UX suggestions from: #472060 (comment 1997342715)

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Setup to simulate SaaS
2. Go to /users/sign_up to register a new user
3. At the welcome page, choose Just me and Join a project
4. And you should land on the project dashboard page with the alert.