[kbn-grid-layout] Fix mobile styles (#212312)

## Summary

This PR fixes a broken style introduced in
https://github.com/elastic/kibana/pull/210285 where, when rewriting the
single column styles from strings to objects, I accidentally set the
property `gridTemplateAreas` instead of `gridTemplateColumns` - this
resulted in the single column / mobile view being broken:

| Before | After |
|--------|--------|
|
![image](https://github.com/user-attachments/assets/e0541cc7-66dd-41a0-8e21-6931c491cea4)
|
![image](https://github.com/user-attachments/assets/12bc1453-1da4-4784-9242-b1cccdf9e24e)
|


### Checklist

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

src/platform/packages/private/kbn-grid-layout/grid/grid_layout.tsx

@@ -181,7 +181,7 @@ const styles = {
   singleColumn: css({
     '&.kbnGrid--mobileView': {
       '.kbnGridRow': {
-        gridTemplateAreas: '100%',
+        gridTemplateColumns: '100%',
         gridTemplateRows: 'auto',
         gridAutoFlow: 'row',
         gridAutoRows: 'auto',