Add pages deployments limit for `opensource` plan

In !149161 (merged) we've introduced the plan limits for Pages Parallel deployments. However we didn't include a limit for opensource, which means that plan defaults to 0 parallel deployments.

Per https://handbook.gitlab.com/handbook/marketing/developer-relations/community-programs/open-source-program/#what-are-the-benefits-of-the-gitlab-for-open-source-program we should provide the same limits as the ultimate subscription.