Project cannot belong to bot user namespace

What does this MR do and why?

• Adding validation at the ActiveRecord level to prevent creation of projects within bot user namespaces via the Projects API
• #362683 (closed)

Screenshots or screen recordings

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How to set up and validate locally

1. Create a group access token
2. Use the group access token to create a project via the API (do not pass in namespace_id param so that the fallback is to create the project within the bot user's namespace)

   curl --request POST --header "PRIVATE-TOKEN: $TOKEN" \
    --header "Content-Type: application/json" --data '{
       "description": "New Project", "path": "bot_project_3",
       "initialize_with_readme": "true"}' \
    --url 'http://localhost:3000/api/v4/projects/'

3. An error should be returned indicating that a project can't be created in a bot user's namespace.
4. You can also pull the namespace_id of the bot user from Rails console and try to pass that to curl:

   rails c
   User.last.namespace.id # because my last action was creating a group access token the last user is the group access token bot user
     => 77
   
   curl --request POST --header "PRIVATE-TOKEN: $TOKEN" \
    --header "Content-Type: application/json" --data '{
       "description": "New Project", "path": "bot_project_4",
       "namespace_id": "77", "initialize_with_readme": "true"}' \
    --url 'http://localhost:3000/api/v4/projects/'

5. An error should be returned indicating that a project can't be created in a bot user's namespace.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.