DOCS GraphQL v4 changelog note (#9762)

Context: https://github.com/silverstripe/silverstripe-graphql/issues/282

docs/en/04_Changelogs/4.8.0.md

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+# 4.8.0 (Unreleased)
+
+## Overview
+
+- [Support for silverstripe/graphql v4](#graphql-v4)
+
+## New features
+
+### Support for silverstripe/graphql v4 {#graphql-v4}
+
+The [silverstripe/graphql](http://github.com/silverstripe/silverstripe-graphql/issues) module
+to drive various parts of the CMS UI, as well as building your own GraphQL APIs in Silverstripe projects.
+
+The CMS currently ships with `silverstripe/graphql:^3`, which is stable but slow for larger schemas.
+The module was rewritten from scratch with performance in mind
+(see [goals](https://github.com/silverstripe/silverstripe-framework/issues/8824) and [pull request](https://github.com/silverstripe/silverstripe-graphql/pulls)).
+The resulting`silverstripe/graphql:^4` release is currently in a alpha stage.
+We encourage everyone to get started, read our new [GraphQL documentation](https://docs.silverstripe.org/en/4/developer_guides/graphql/), 
+provide [feedback](https://github.com/silverstripe/silverstripe-graphql/issues) and help us stabilise the module.
+
+In order to facilitate the new module release, most GraphQL helper classes to drive the CMS UI
+have been marked as deprecated, and moved to `GraphQL/_legacy` folders within core modules.
+The GraphQL schema under `admin/graphql` has been altered slightly to enable forwards compability,
+with lower-camelcase field names, and a different filter argument structure.
+In the unlikely case that you are relying on those classes or queries directly,
+consider upgrading to `silverstripe/graphql:^4`.
+Your own GraphQL schemas outside of `admin/graphql` should not be affected by the changes
+if you choose to stay on `silverstripe/graphql:^3` for the time being.
+
+We are planning to require the use of `silverstripe/graphql:^4` in a future CMS 4.x release.