Allow to toggle requirements_access_level on REST API

What does this MR do and why?

Allow to set or update project requirements_access_level on API.

Our REST API docs shows that it is possible to use the attribute, however it was missing from code.

related to #323886 (closed)

How to set up and validate locally

When creating a project

• curl --request POST "https://gitlab/api/v4/projects?name=<example-name>&requirements_access_level=disabled"

When updating a project

curl --request PUT --header "PRIVATE-TOKEN: <your-token>" \
     --url 'https://gitlab.com/api/v4/projects/<your-project-ID>' \
     --data "requirements_access_level=disabled" 

MR acceptance checklist

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