This commit adds a runner minutes cost factor for public projects in new
namespaces. "New" is defined as "after July 17, 2021".

* Renames Ci::Runner#minutes_cost_factor to #cost_factor_for_project and
  updates it to take a project
* Adds Ci::Minutes::CostFactor#for_project

https://gitlab.com/gitlab-org/gitlab/-/issues/330581

Changelog: changed
EE: true