Add opensource plan to database

What does this MR do and why?

The MR creates a plan opensource titled 'Open Source Program' within the plans table. The opensource plan is used to differentiate between the free/ultimate plans and open source program plans and would have different limits.

How to set up and validate locally

1. Install GDK.
2. Checkout the branch in gitlab instance 343134-create-open-source-plan
3. Run rails db:migrate
4. Check plans table within database and should have new plan opensource

Database migrations

rails db:migrate
== 20211029102822 AddOpenSourcePlan: migrating ================================
== 20211029102822 AddOpenSourcePlan: migrated (0.0728s) =======================

rails db:rollback 
== 20211029102822 AddOpenSourcePlan: reverting ================================
== 20211029102822 AddOpenSourcePlan: reverted (0.0125s) =======================

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 #343134 (closed)