Create new Open Source Program plan

Why are we doing this work

As part to distinguish Free plan from Open Source Program, a new plan needs to be created with GitLab plans table.

When a subscription is created with an OpenSource plan within CustomersDot, the CustomersDot should sync the new opensource plan information to the namespace.

This issue would introduce new plan opensource and related plan_limits entries.

Relevant links

The MR here can be looked for reference.

Implementation plan

1. Create migration file to insert new plan opensource
   • title - Open Source Program
   • name - opensource
2. Create migration to add plan_limits for new plan
   • The plan_limits for opensource is a clone of the entries of the ultimate plan.