chore(#11112): bump CI nginx to 1.31

[mrjones-plip]

Description

@sugat009 - me again with another nginx bump! Same as before, I tested by using npm run build-images locally and then running the resulting nginx in a docker helper instance. All seemed to work!

closes #11112

Code review checklist

• Readable: Concise, well named, follows the style guide
• Documented: Configuration and user documentation on cht-docs
• Tested: Unit and/or e2e where appropriate
• Backwards compatible: Works with existing data and configuration or includes a migration. Any breaking changes documented in the release notes.
• AI disclosure: Please disclose use of AI per the guidelines.

Compose URLs

If Build CI hasn't passed, these may 404:

• Core
• CouchDB Single
• CouchDB Cluster

License

The software is provided under AGPL-3.0. Contributions to this project are accepted under the same license.