chore/fix: Removed ChatGPTService and so fixed the `/` root URL path demo (filling the from from text), now all examples should be good.

Description

The text-davinci-003 model on the CompletionRequest (@POST("/v1/completions")) endpoint have been deprecated. Removing it appears to be a solution for now, and it is recommended to use the alternative service, ChatGPTChatCompletionService.

This action is important, because we have had Discord users who were either unable to use the addon or were confused by it because of this issue.

We attempted to use the following models to replace the old text-davinci-003:

davinci-002
babbage-002

Unfortunately, neither of them was able to return a JSON response.

Now all demos shall work:

/invoice (tested)
/receipt (tested)
/ (text to fill, which was broken, but works now)
/camera (this as well, although not tested from mobile)

Fixes # (issue)

https://github.com/vaadin/form-filler-addon/issues/148

Type of change

[x] Bugfix
[ ] Feature

Checklist

[x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
[x] I have added a description following the guideline.
[x] The issue is created in the corresponding repository and I have referenced it.
[x] I have added tests to ensure my change is effective and works as intended.
[x] New and existing tests are passing locally with my change.
[x] I have performed self-review and corrected misspellings.

Additional for `Feature` type of change

[ ] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

vaadin / form-filler-addon