Redirect to unified SaaS and SM purchase flow when visiting old purchase flow

Objective

See the objective outlined in the parent epic [Unify purchase flows] SaaS/Self-Managed Premiu... (&18567).

Purpose

This issue is to redirect customers to unified SaaS and SM purchase flow when visiting the existing purchase flow using the feature flag.

This should also support any valid params that are applicable for SaaS and SM purchase flow to the unified version for parity with current functionality. Example: promo_code param for SaaS and SM purchase flow, add_on_type and add_on_quantity for SaaS purchase flow, anything other param that we currently support in the current SaaS and SM purchase flows.