[Vega] Improve contrast ratio for vega visualizations tooltip key text (

elastic#200208)

## Summary

Closes elastic#151923

We need to improve the contrast ratio in vega kibana tooltip key text.
The current contrast ratio between the background (#404040) and the text
color (#98A2B3) is below 4.5:1 and fails WCAG SC 1.4.3 Contrast
(Minimum).

In order to solve this issue, we have updated the color used for the
**key value** the to use `colors.lightShade` (#D3DAE6). With this
change, we have increased the contrast ratio from 4.02 to 7.37.

You can see the available shades
[here](https://eui.elastic.co/#/theming/colors/values#shades).

### Screenshots

#### Before - contrast ratio 4.02

![Screenshot 2024-11-14 at 16 46
13](https://github.com/user-attachments/assets/19c22538-6540-4e6a-909d-8abd58facbbc)


#### After  - contrast ratio 7.37

![Screenshot 2024-11-14 at 16 44
43](https://github.com/user-attachments/assets/a3ca319f-0cfa-4e79-a4ae-fddd64fc2e42)


### Checklist

Check the PR satisfies following conditions. 

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

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

src/plugins/vis_types/vega/public/vega_view/vega_tooltip.styles.ts

@@ -53,7 +53,7 @@ export const vegaVisTooltipStyles = (euiThemeContext: UseEuiTheme) => {
             'max-width',
             mathWithUnits(euiTheme.size.base, (x) => x * 10)
           )}
-          color: ${euiTheme.colors.mediumShade};
+          color: ${euiTheme.colors.lightShade};
           ${logicalTextAlignCSS('right')}
           ${logicalCSS('padding-right', euiTheme.size.xs)}
         }