[Production rollout] Update CDot documentation
### Summary Once we are preparing for the production rollout we want to remove from the [CustomersDot temporary extension documentation](https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/main/doc/flows/temporary_extensions.md) references to the feature flag and the new and old ways of processing these. ### Acceptance criteria - Update the documentation so it only describes the process and conditions for when this associated FF is enabled - Include the [guardrails specifics](`https://gitlab.com/gitlab-org/fulfillment-meta/-/issues/997#guardrails-and-abuse-prevention`) under the "Guardrail checks" service and make sure it is up to date: - No restriction of one extension per subscription term - Includes the Salesforce Approval requirement
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