Bump fastapi from 0.94.1 to 0.104.1

What does this merge request do and why?

This MR bumps fastapi from 0.94.1 to 0.104.1. Also fix request body declaration as per https://github.com/tiangolo/fastapi/discussions/9287. This version of fastapi still supports Pydantic v1 so I will hold off Pydantic v2 upgrade to another MR.

• Minor version bump changelog
• Full changelog.

How to set up and validate locally

1. Check out to this merge request's branch.
2. Ensure a local Docker image built successfully.

   docker buildx build --platform linux/amd64 \
     -t api-gateway:dev .

3. Run a local service on Docker.

   docker run --platform linux/amd64 --rm \
     -p 5052:5052 \
     -e AUTH_BYPASS_EXTERNAL=true \
     -v $PWD:/app -it api-gateway:dev

4. Ensure the app boots up correctly.
5. Send a few test cURL requests and confirm they work.

   curl --request POST \
     --url http://localhost:5052/v2/completions \
     --header 'Content-Type: application/json' \
     --header 'X-Gitlab-Authentication-Type: oidc' \
     --data '{
     "prompt_version": 1,
     "project_path": "awesome_project",
     "project_id": 23,
     "current_file": {
       "file_name": "main.ts",
       "content_above_cursor": "\nfunction calculator(a: Number, b: Number, operation: string): number {\n    // operation can be +, -, * or /\n    \n",
       "content_below_cursor": ""
     }
   }'

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.