Backend: Use a reduced cost factor for contributions to OSS plan projects

Problem

After #346400 (closed) we would like to enforce CI minutes limits more by reducing the scope of projects that is eligible for the reduced cost factor.

Proposal

in Ci::Minutes::CostFactor class:

• Narrow the definition of OSS contribution as: a pipeline running for a public fork of a public upstream project in the OSS plan.
  • For any jobs classified as OSS contributions, use a cost factor 0.008 which was introduced/separated-out in #346399 (closed)
  • This ensures that the users can contribute up to 50,000 minutes (duration) on OSS plan projects (400 CI minutes limit for Free plan / cost factor 0.008 = 50,000).