Update test_post_error

The `log_output` fixture (`tests/conftest.py`) returns an instance of
`structlog.testing.LogCapture`. structlog v24.3.0 changed the behaviour
of `LogCapture`, via the `map_method_name` function, to map the
`exception` log level to the `error` log level (hynek/structlog#628).
This change broke `test_post_error`. Here, we simply accept that
decision, and update `test_post_error`.

tests/unit/services/test_slack.py

@@ -28,7 +28,7 @@ def test_post_error(mocker, log_output):
         "channel": "channel",
         "exc_info": True,
         "event": "Failed to notify slack in channel: channel",
-        "log_level": "exception",
+        "log_level": "error",
     }