[ML] Fix accessibility warnings in notifications and memory usage cha…

…rt (#153876)

## Summary

Fixes a couple of accessibility warnings - one for the unread
notifications indicator in the sidebar nav, and one on the indicator for
the maximum memory for ML native process on the memory usage chart.

#### Unread notifications indicator
Fix adds a `role` attribute which is needed when `aria-label` is used on
a `<div>`.
<img width="194" alt="image"
src="https://user-images.githubusercontent.com/7405507/228281236-0aeebb3c-8726-42ed-895f-88f2d614dc7b.png">

#### Memory usage chart
Fix adds an `aria-label` attribute to the max memory `EuiIcon` which is
rendered as a `button`.
<img width="437" alt="image"
src="https://user-images.githubusercontent.com/7405507/228281304-b42e7dcf-ecc2-45e3-8b08-a8ab43ff5f02.png">


### Checklist

- [x] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))

Fixes #153592
Fixes #153596

Co-authored-by: Kibana Machine <[email protected]>

x-pack/plugins/ml/public/application/components/ml_page/notifications_indicator.tsx

@@ -76,6 +76,7 @@ export const NotificationsIndicator: FC = () => {
             <EuiHealth
               css={{ display: 'block' }}
               color="primary"
+              role="img"
               aria-label={i18n.translate('xpack.ml.notificationsIndicator.unreadIcon', {
                 defaultMessage: 'Unread notifications indicator.',
               })}

x-pack/plugins/ml/public/application/memory_usage/nodes_overview/memory_preview_chart.tsx

@@ -138,7 +138,15 @@ export const MemoryPreviewChart: FC<MemoryPreviewChartProps> = ({ memoryOverview
             }),
           },
         ]}
-        marker={<EuiIcon type="arrowDown" />}
+        marker={
+          <EuiIcon
+            type="arrowDown"
+            aria-label={i18n.translate('xpack.ml.trainedModels.nodesList.mlMaxMemoryAriaLabel', {
+              defaultMessage: 'Maximum memory permitted for ML native processes {bytes}',
+              values: { bytes: bytesFormatter(memoryOverview.ml_max_in_bytes) },
+            })}
+          />
+        }
         markerPosition={Position.Top}
       />