docs: Removal of Subnet

Author: mdbooth

docs/book/src/topics/crd-changes/v1alpha6-to-v1alpha7.md

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+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*
+
+- [v1alpha6 compared to v1alpha7](#v1alpha6-compared-to-v1alpha7)
+  - [Migration](#migration)
+  - [API Changes](#api-changes)
+    - [`OpenStackCluster`](#openstackcluster)
+    - [`OpenStackMachine`](#openstackmachine)
+      - [Removal of Subnet](#removal-of-subnet)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+# v1alpha6 compared to v1alpha7
+
+> ⚠️ v1alpha7 has not been released yet.
+
+## Migration
+
+All users are encouraged to migrate their usage of the CAPO CRDs from older versions to `v1alpha6`. This includes yaml files and source code. As CAPO implements automatic conversions between the CRD versions, this migration can happen after installing the new CAPO release.
+
+## API Changes
+
+This only documents backwards incompatible changes. Fields that were added to v1alpha6 are not listed here.
+
+### `OpenStackCluster`
+
+### `OpenStackMachine`
+
+#### Removal of Subnet
+
+The OpenStackMachine spec previously contained a `subnet` field which could used
+to set the `accessIPv4` field on Nova servers. This feature was not widely
+used, difficult to use, and could not be extended to support IPv6. It is
+removed without replacement.