Remove ci_cost_factors_narrow_os_contribution_by_plan flag

What does this MR do and why?

Remove the ci_cost_factors_narrow_os_contribution_by_plan and document narrowing the scope of what is considered an open source contribution.

How to set up and validate locally

Example below:

1. Create a public project
2. Add it to the open source plan

   To do

3. Fork the project
4. Trigger a job with shared runners with a sleep script.
5. Cancel the job after a few minutes.
6. Check logs for .008 cost factor

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 #372263 (closed)