bump image ver to 3.20

[gillettmoj]

Purpose

Bumps php image from php:8.2-fpm-alpine3.18 to php:8.2-fpm-alpine3.20

Fixes LPAL-1304

Approach

Explain how your code addresses the purpose of the change

Learning

Any tips and tricks, blog posts or tools which helped you. Plus anything notable you've discovered about the LPA service

Checklist

• I have performed a self-review of my own code
• I have updated documentation (Confluence/GitHub wiki/tech debt doc) where relevant
• I have added tests to prove my work
• I have added mandatory tags to terraformed resources, where possible
• If I have a new OPG component dependency, I have updated the metadata.json with the repo location.
• If I added a package.json or composer.json, I also made sure this is included in the script in .github/workflows/dependabot-update.yml
• The product team have tested these changes