Update Zoekt Production Readiness Review to reflect larger scale

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The readiness review done in gitlab-com/gl-infra/readiness!146 (merged) was designed to reflect the readiness of running Zoekt at a smaller scale for a very limited number of groups. There were many sections that had statements like "we plan to ..." in it. We should make sure we update this document to reflect that all these sections are now actually completed before we roll out to a larger scale of customers.