Add pages deployments limit for opensource plan

What does this MR do and why?

This MR introduces a active_versioned_pages_deployments_limit_by_namespace plan limit amount for opensource plans. The value is set to the same amount as ultimate, as they should align. The opensource plan has been accidentally ommitted when the plan was originally created in !149161 (merged)

Screenshots or screen recordings

No UI Changes

How to set up and validate locally

1. Run bin/rails db:migrate
2. Open the rails console with rails c
3. Run Plan.find_by(name: "opensource").limits.active_versioned_pages_deployments_limit_by_namespace and verify the result is now 500 – the same as Plan.find_by(name: "ultimate").limits.active_versioned_pages_deployments_limit_by_namespace.

Related to #500049 (closed)