Upgrade API to Python 3.12

[sarayourfriend]

Current Situation

The API runs Python 3.11. It should run the latest Python.

Suggested Improvement

Update the API to use 3.12:

• Update api/pyproject.toml to use 3.12
• Update the API Dockerfile to use 3.12

Benefit

Use the latest available Python. Don't fall behind and get hit with surprising migration issues when the next major versions roll around, or when Django et co drop support for 3.11.