DDLS-399 upgrade default, integration and pre rds to 16.4

[jamesrwarren]

Purpose

Upgrade RDS postgres version to 16.4

Fixes DDLS-399

Approach

Part 1 of 2. This one is to upgrade the integration and preproduction versions. Second part will do production once we are happy preprod is working fine and had no issues upgrading.

Learning

NA

Checklist

• I have performed a self-review of my own code
• I have updated documentation (Confluence/ADR/tech debt doc) where relevant
• I have added tests to prove my work
• The product team have approved these changes
• I have checked my work for potential security issues and refered to the OWASP top 10

Frontend

• I have run an in-browser accessibility test (e.g. WAVE, Lighthouse)
• There are no deprecated CSS classes noted in the profiler
• Translations are used and the profiler doesn't identify any missing
• Any links or buttons added are screen reader friendly and contextually complete
• If adding GA events, I have updated or checked the existing category or label values