migrations: use startup migration for tenants' system.zones

[irfansharif]

Part of #66348. In #67323 we introduced per-tenant system.zones using tenant migrations for extant tenants, and bootstrap code for new tenants. Startup migrations (today called sqlmigrations) seem better suited for introducing new system tables + populating it with initial static values.

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[irfansharif]

Not doing this, keeping it as a tenant migration + bootstrap. #69169.