Add note about network breadcrumb metadata changes to UPGRADING.md

UPGRADING.md

@@ -53,6 +53,10 @@ Breadcrumb support has been enabled for node. This means you can call `Bugsnag.l
 
 Currently no breadcrumbs are automatically collected in node.
 
+#### `request` replaced with `url` and `method` in network breadcrumb metadata
+
+Prior to v8, network breadcrumb metadata included a field named `request`, which contained the request URL prepended with the HTTP method (e.g. `"GET https://request-url.com/`). This has been replaced with two separate metadata fields named `url` and `method`, which contain the request URL and HTTP method respectively.
+
 ## `bugsnag-react-native@*` to `@bugsnag/[email protected]`
 
 As of `v7.3` of the [`bugsnag-js` monorepo](https://github.com/bugsnag/bugsnag-js) it contains Bugsnag's SDK for React Native. This additional notifier joins `@bugsnag/js` and `@bugsnag/expo` in its unified version scheme, so the first version of `@bugsnag/react-native` is `v7.3.0`.