Fix preselected namespace when creating a project

What does this MR do?

It fixes the preselected namespace on the projects/new page with a given namespace_id in params.

Currently, the wrong namespace gets selected when the user tries to create a project and gets a validation error. In this case the namespace of the current user gets selected (see Issue #26879 (closed)).

This is because the params hash in the controller changes and the provided namespace_id is then in a project subhash (e.g. params[:project][:namespace_id] instead of params[:namespace_id]).

Why was this MR needed?

In order to fix fetching the namespace_id from the params hash.

Open questions

The current solution will look up the namespace_id in the project subhash first and then in params. If it's present in both the project subhash has priority. But I think this shouldn't be a problem.

I'm not sure if I can already use the Hash#dig method here.

What are the relevant issue numbers?

Closes #26879 (closed)