Add blog post about deprecating async_track_state_change (#2144)

Co-authored-by: Jan-Philipp Benecke <github@bnck.me>

blog/2024-04-13-deprecate_async_track_state_change.md

@@ -0,0 +1,40 @@
+---
+author: J. Nick Koston
+authorURL: https://github.com/bdraco
+title: "Replacing `async_track_state_change` with `async_track_state_change_event`"
+---
+
+`async_track_state_change` is deprecated and will be removed in Home Assistant 2025.5. `async_track_state_change_event` should be used instead.
+
+`async_track_state_change` always creates a top-level listener for `EVENT_STATE_CHANGED`, which would have to reject all state changes that did not match the desired entities. This design presented a performance problem when there were many integrations using `async_track_state_change`. `async_track_state_change` has been phased out in `core` since the introduction of `async_track_state_change_event`, with the last instance being removed in 2024.5.
+
+Example with `async_track_state_change`:
+
+```python
+from homeassistant.core import State, callback
+from homeassistant.helper.event import async_track_state_change
+
+@callback
+def _async_on_change(entity_id: str, old_state: State | None, new_state: State | None) -> None:
+    ...
+
+unsub = async_track_state_change(hass, "sensor.one", _async_on_change)
+unsub()
+```
+
+Example replacement with `async_track_state_change_event`:
+
+```python
+from homeassistant.core import Event, EventStateChangedData, callback
+from homeassistant.helper.event import async_track_state_change_event
+
+@callback
+def _async_on_change(event: Event[EventStateChangedData]) -> None:
+    entity_id = event.data["entity_id"]
+    old_state = event.data["old_state"]
+    new_state = event.data["new_state"]
+    ...
+
+unsub = async_track_state_change_event(hass, "sensor.one", _async_on_change)
+unsub()
+```