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

[ciscoo]

🤔 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