docs(server): correct Tink key rotation and fix misleading cannot be …

[denislemire]

…changed wording

• Remove incorrect claim that Tink/Vault cannot be changed after deployment
• State that Tink keys can be rotated; link to support article
• Add link to server-scripts vault-to-tink for Vault to Tink migration
• Update phase-2 and values/installation-reference across Server 4.2–4.9

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