Revise changelog entries for service settings refactor

Split the single "changed" entry into separate "added", "changed", and "deprecated" entries for clarity. Add a note about the subtle behavior change in the deprecated set_model/set_voice/set_language methods.

changelog/3714.added.md

@@ -0,0 +1,19 @@
+- Added support for using strongly-typed objects instead of dicts for updating service settings at runtime.
+
+  Instead of, say:
+
+  ```python
+  await task.queue_frame(
+      STTUpdateSettingsFrame(settings={"language": Language.ES})
+  )
+  ```
+
+  you'd do:
+
+  ```python
+  await task.queue_frame(
+      STTUpdateSettingsFrame(update=DeepgramSTTSettings(language=Language.ES))
+  )
+  ```
+
+  Each service now vends strongly-typed classes like `DeepgramSTTSettings` representing the service's runtime-updatable settings.