Add s3 warning to changelog and upgrade notes

CHANGES.md

@@ -1,5 +1,14 @@
 # Synapse 1.140.0rc1 (2025-10-10)
 
+## Compatibility notice for users of `synapse-s3-storage-provider`
+
+Deployments that make use of the
+[synapse-s3-storage-provider](https://github.com/matrix-org/synapse-s3-storage-provider)
+module must upgrade to
+[v1.6.0](https://github.com/matrix-org/synapse-s3-storage-provider/releases/tag/v1.6.0).
+Using older versions of the module with this release of Synapse will prevent
+users from being able to upload or download media.
+
 ## Features
 
 - Add [a new Media Query by ID Admin API](https://element-hq.github.io/synapse/v1.140/admin_api/media_admin_api.html#query-a-piece-of-media-by-id) that allows server admins to query and investigate the metadata of local or cached remote media via

docs/upgrade.md

@@ -117,6 +117,16 @@ each upgrade are complete before moving on to the next upgrade, to avoid
 stacking them up. You can monitor the currently running background updates with
 [the Admin API](usage/administration/admin_api/background_updates.html#status).
 
+# Upgrading to v1.140.0
+
+## Users of `synapse-s3-storage-provider` must update the module to `v1.6.0`
+
+Deployments that make use of the
+[synapse-s3-storage-provider](https://github.com/matrix-org/synapse-s3-storage-provider/)
+module must update it to
+[v1.6.0](https://github.com/matrix-org/synapse-s3-storage-provider/releases/tag/v1.6.0),
+otherwise users will be unable to upload or download media.
+
 # Upgrading to v1.139.0
 
 ## `/register` requests from old application service implementations may break when using MAS