[Data] Fix Task Completion Time Without Backpressure Grafana panel metric name (ray-project#60481)

## Description
Fixes the broken **"Task Completion Time Without Backpressure"** metrics
chart in the Ray Data Grafana dashboard.

The panel was querying
`ray_data_task_completion_time_without_backpressure`, which no longer
exists. PR ray-project#57788 renamed the underlying metric to
`task_completion_time_excl_backpressure_s` in `op_runtime_metrics.py`,
but the Grafana panel in `data_dashboard_panels.py` was not updated.
This PR updates the panel's Prometheus `expr` to use
`ray_data_task_completion_time_excl_backpressure_s` so the chart
displays data again.

**Change:** Single-line fix in `data_dashboard_panels.py` — replace the
old metric name with the correct one in the panel's `expr`. The formula
(average task completion time excluding backpressure over a 5-minute
window) is unchanged.

## Related issues
Fixes the regression from ray-project#57788 (metric rename). Related to Ray Data
monitoring / dashboard.
Closes: ray-project#60163

## Additional information
- **Metric flow:**
`op_runtime_metrics.task_completion_time_excl_backpressure_s` → Stats
uses `data_{name}` → Metrics agent adds `ray_` namespace →
**`ray_data_task_completion_time_excl_backpressure_s`**
- **Manual verification:** Run a Ray Data job with Grafana + Prometheus
(see [cluster
metrics](https://docs.ray.io/en/latest/cluster/metrics.html)), then
confirm the "Task Completion Time Without Backpressure" panel shows
data.

Signed-off-by: kriyanshii <kriyanshishah06@gmail.com>
Signed-off-by: Adel Nour <ans9868@nyu.edu>

Author: kriyanshii

python/ray/dashboard/modules/metrics/dashboards/data_dashboard_panels.py

@@ -684,7 +684,7 @@
     unit="s",
     targets=[
         Target(
-            expr='increase(ray_data_task_completion_time_without_backpressure{{{global_filters}, operator=~"$Operator"}}[5m]) / increase(ray_data_num_tasks_finished{{{global_filters}, operator=~"$Operator"}}[5m])',
+            expr='increase(ray_data_task_completion_time_excl_backpressure_s{{{global_filters}, operator=~"$Operator"}}[5m]) / increase(ray_data_num_tasks_finished{{{global_filters}, operator=~"$Operator"}}[5m])',
             legend="Task Completion Time w/o Backpressure: {{dataset}}, {{operator}}",
         ),
     ],