Add 2.63 breaking incompatibility to CHANGES.md (#34931)

* Add 2.63 breaking incompatibility to CHANGES.md

My change in Coder #33626 has introduced breaking change leading to Coder serial version to change. This mostly affects Coders extending CustomCoder and Coder.

* Update CHANGES.md

* Update CHANGES.md

Author: stankiewicz

CHANGES.md

@@ -253,6 +253,7 @@ N/A
 ## Known Issues
 
 * (Java) Current version of protobuf has a [bug](https://github.com/protocolbuffers/protobuf/issues/20599) leading to incompatibilities with clients using older versions of Protobuf ([example issue](https://github.com/GoogleCloudPlatform/DataflowTemplates/issues/2191)). This issue has been seen in SpannerIO in particular. Tracked in [#34452](https://github.com/GoogleCloudPlatform/DataflowTemplates/issues/34452).
+* (Java) Coders that extend CustomCoder (excluding SchemaCoder and RowCoder) are update incompatible with previous versions of the same transform (Java) on some runners. This includes PubSubIO's read ([#34933](https://github.com/apache/beam/issues/34933)).
 
 # [2.62.0] - 2025-01-21