Move limits from Plan to new PlanLimits model

What does this MR do?

This MR is targeting add-defaul-plan because it needs changes defined in !19033 (merged)

In this MR we are moving columns related to CI limits from Plan into a PlanLimits model. This refactoring allows us in the future to define namespace-level limits with the same table schema of PlanLimits and have:

class Namespace
  def actual_limit
    limits || actual_plan.limits
  end
end

Also, this means we can add more limits to this table without polluting the Plan's responsibilities.

Related issue: #32823 (closed)

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