Mention ISummarizerRuntime deprecated methods (#12702)

[How contribute to this
repo](https://github.com/microsoft/FluidFramework/blob/main/CONTRIBUTING.md).

[Guidelines for Pull
Requests](https://github.com/microsoft/FluidFramework/wiki/PR-Guidelines#guidelines).

## Description

These APIs were deprecated in
[#9957](#9957) but
weren't mentioned in BREAKING.md

BREAKING.md

@@ -15,6 +15,17 @@ It's important to communicate breaking changes to our stakeholders. To write a g
 - Avoid using code formatting in the title (it's fine to use in the body).
 - To explain the benefit of your change, use the [What's New](https://fluidframework.com/docs/updates/v1.0.0/) section on FluidFramework.com.
 
+# 2.0.0-internal.2.1.0
+
+## 2.0.0-internal.2.1.0 Upcoming changes
+- [Deprecated ISummarizerRuntime batchEnd listener](#Deprecated-ISummarizerRuntime-batchEnd-listener)
+
+### Deprecated ISummarizerRuntime batchEnd listener
+The `"batchEnd"` listener in `ISummarizerRuntime` has been deprecated and will be removed in a future release. Please remove all usage and implementations of `ISummarizerRuntime.on("batchEnd", ...)` and `ISummarizerRuntime.removeListener("batchEnd", ...)`.
+If these methods are needed, please refer to the `IContainerRuntimeBase` interface.
+
+## 2.0.0-internal.2.1.0 Breaking changes
+
 # 2.0.0-internal.2.0.0
 
 ## 2.0.0-internal.2.0.0 Upcoming changes