Allow only public projects while creating a workspace

MR: Workspace validates its project is public (!118989 - merged)

Problem to solve

If we try to create a workspace from a private repository, the cloning of the project fails and the workspace never reaches a running state.

Proposal

Only allow creating workspaces from a public repository until we can support cloning private repositories.

Document this limitation clearly with information about the workaround.

But wait, that's a big limitation

The workaround for this is to create a workspace from a public repository first (which doesn't actually have to contain any code) then authenticate inside the workspace and clone a private repository manually. This is a major point of UX friction, but one we will address in Credentials are injected to clone a private Git... (&10264 - closed)