chore: mark fetch-error-handler as stable

This library has been used in production for months and is now serving
as a replacement for the deprecated `n-fetch` and `n-eager-fetch`
libraries.

There have been no reported issues, it's installed in 9 production
systems (including content pipeline), and we're seeing ~2m errors per
week in Splunk that were successfully handled by this library:

```
index IN (heroku, aws_cloudwatch)
(error.code=FETCH_* OR error.cause.code=FETCH_*)
```

Release-As: 1.0.0

Author: rowanmanning

packages/fetch-error-handler/README.md

@@ -3,9 +3,6 @@
 
 Properly handle fetch errors and avoid a lot of boilerplate in your app. This module is part of [FT.com Reliability Kit](https://github.com/Financial-Times/dotcom-reliability-kit#readme).
 
-> [!WARNING]<br />
-> This package is in beta and hasn't been tested extensively in production yet. Feel free to use, and any feedback is greatly appreciated.
-
   * [Usage](#usage)
     * [Wrap the fetch function](#wrap-the-fetch-function)
     * [Handle errors with `.then`](#handle-errors-with-then)