Allow ci_separated_caches to be updated from API

What does this MR do and why?

This allows the ci_separated_caches property to be updated with the API

How to set up and validate locally

1. Checkout the branch
2. Start GDK

PUT http://127.0.0.1:3000/api/v4/projects/1

{
  "ci_separated_caches": false
}

4. Verify that the ci_separated_caches value is false in the response
5. Verify that Settings > CI/CD > General Pipelines has Use separate caches for protected branches disabled
6. Repeat steps 3 - 5 with the other direction to enable the setting again

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #365225 (closed)