add #29987 to 2.53 known issues in CHANGES.md

[clmccart]

add #29987 to 2.53 known issues in CHANGES.md

---

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[Abacn]

thanks. Is this known issue newly introduced in 2.53.0, or anything need special concern? That section in CHANGES were not quite actively used. It's usually used to add regression of recent version that discovered later.

[clmccart]

thanks. Is this known issue newly introduced in 2.53.0, or anything need special concern? That section in CHANGES were not quite actively used. It's usually used to add regression of recent version that discovered later.

this issue is newly introduced in 2.53.0. I reached out to @jrmccluskey to see how best to proceed and he suggested adding this to the known issues section and then adding the fix to the 2.54 milestone

[Abacn]

Suggested change

	* ([#29987](https://github.com/apache/beam/issues/29987)).
	* Potential race condition causing NPE in DataflowExecutionStateSampler on Dataflow Streaming ([#29987](https://github.com/apache/beam/issues/29987)).

some brief description