Built-in reporter plugins section should mention JSON format is in maintenance mode

### 🤔 What's the problem you've observed?

The README for https://github.com/cucumber/messages has:

> The `json` formatter is now in maintenance mode for these implementations, and Messages is the preferred standard.

The documenation for Built-in reporter plugins is missing that information: https://cucumber.io/docs/cucumber/reporting/#built-in-reporter-plugins

### ✨ Do you have a proposal for making it better?

Add a note about the `json` formatter being in maintenance mode and to consider using other options.

While maintenance does not necessairly mean it will removed, I think it's good information to include in the documentation.

### 📚 Any additional context?

_No response_

Start at the Built-in reporter plugins section of the Cucumber reporting documentation and compare its wording with the linked cucumber/messages README. Add the note that the json formatter is in maintenance mode and suggest considering other options; done means the published section contains that information accurately.

Written by the indexing model from the issue text.