Add changelog entries for OpenAILLMContext removal

changelog/4215.removed.md

@@ -0,0 +1,18 @@
+- ⚠️ Removed `OpenAILLMContext`, `OpenAILLMContextFrame`, and `OpenAILLMContext.from_messages()`. Use `LLMContext` (from `pipecat.processors.aggregators.llm_context`) and `LLMContextFrame` (from `pipecat.frames.frames`) instead. All services now exclusively use the universal `LLMContext`.
+
+  From the developer's point of view, migrating will usually be a matter of going from this:
+
+  ```python
+  context = OpenAILLMContext(messages, tools)
+  context_aggregator = llm.create_context_aggregator(context)
+  ```
+
+  To this:
+
+  ```python
+  from pipecat.processors.aggregators.llm_context import LLMContext
+  from pipecat.processors.aggregators.llm_response_universal import LLMContextAggregatorPair
+
+  context = LLMContext(messages, tools)
+  context_aggregator = LLMContextAggregatorPair(context)
+  ```