fix: [AXE-CORE][Serverless - Observability] Observability -> SLOs -> …

…Metric sparkline charts fail <svg> elements with an img role must have an alternative text a11y check (#183040)

Closes: #179491

## Summary

Observability ->SLOs page -> Metric sparkline charts fail <svg> elements
with an img role must have an alternative text a11y check.

## What was done:? 
1. The `trendA11yTitle` attribute has been added within the
`SloCardChart` component to address accessibility concerns

## Screen: 

### AXE report: 

![image](https://github.com/elastic/kibana/assets/20072247/e366fcf8-fe4b-40a8-baf3-366003b3d16c)

### A11Y tree:

![image](https://github.com/elastic/kibana/assets/20072247/d2633990-a089-477c-8fe1-2d8fefadc8fd)

x-pack/plugins/observability_solution/slo/public/pages/slos/components/card_view/slo_card_item.tsx

@@ -244,6 +244,12 @@ export function SloCardChart({
               title: slo.name,
               subtitle: subTitle,
               value: sliValue,
+              trendA11yTitle: i18n.translate('xpack.slo.slo.sLOGridItem.trendA11yLabel', {
+                defaultMessage: `The "{title}" trend`,
+                values: {
+                  title: slo.name,
+                },
+              }),
               trendShape: MetricTrendShape.Area,
               trend: historicalSliData?.map((d) => ({
                 x: d.key as number,