feat: remove code gecko as default code completion

What does this merge request do and why?

Once we are ready, deprecate code-gecko as the default code completion model and replace it with Codestral 25.01

How to set up and validate locally

curl -X "POST" "http://localhost:5052/v2/code/completions" \
     -H 'Content-Type: application/json; charset=utf-8' \
     -d $'{
  "prompt_version": 1,
  "current_file": {
    "content_below_cursor": "pageSource = document.querySelector(\'#hidden-page-source\').href\\n}, false);",
    "file_name": "main.go",
    "language_identifier": "go",
    "content_above_cursor": "document.addEventListener(\'DOMContentLoaded\', function() {\\n  webIDE = document.querySelector("
  }
}'

• For AIGW_GOOGLE_CLOUD_PLATFORM__LOCATION=asia-east1 should use Fireworks/Qwen
• For AIGW_GOOGLE_CLOUD_PLATFORM__LOCATION=us-central1 should use Vertex/Codestral25.01

Resolves gitlab-org/gitlab#519375 (closed)

Merge request checklist

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