Fix replicated default configuration

[linear]

The default configuration for the backend application is defined and replicated in various places:

• https://github.com/Significant-Gravitas/AutoGPT/blob/master/autogpt_platform/backend/config.default.json#L1
• https://github.com/Significant-Gravitas/AutoGPT/blob/master/autogpt_platform/backend/backend/util/settings.py#L57
• https://github.com/Significant-Gravitas/AutoGPT/blob/master/autogpt_platform/backend/.env.example#L13
• https://github.com/Significant-Gravitas/AutoGPT/blob/master/autogpt_platform/docker-compose.platform.yml#L136

The default value can all be defined in a single place (settings.py default value). The rest can be defined only if it has to be a strict environment variable or a customization is required between different environments. (this is where docker env, dot env, config.env.json) comes in.