Add subsidized cost factor for Open Source plan

What does this MR do and why?

• Related to #346399 (closed) (main issue)
• Related to #343134 (closed) (related because I've added Open Source program to the list of SaaS plans in Plan model)

With the new Open Source plan been added specifically for those namespaces member of the Open Source program we are now able to distinguish open source projects from those in either Free or paid plans.

As we aim to harden CI minutes limits for all public projects we need to first ensure we don't impact public projects that are part of the Open Source program. For this reason, in this MR we are lowering the cost factor to 0.008 for those open source projects, making the Open Source program more appealing for open source projects and ensuring we don't impact them when we are going to raise the cost factor to 1.0 for public projects in upcoming milestones.

Lowering the cost factor means that CI minutes consumption moves at a slower rate (currently 125x slower than on private projects), effectively allowing a namespace to use more CI minutes. The limit cost factor 0.008 is low enough to ensure that a namespace can use 125x more CI minutes than their monthly allowance, but it'a also a non-zero value which means that the consumption is still capped.

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