fix issue with font size in markdown component (#179867)

## Summary

Closes #179611

## Visual confirmation
![ScreenRecording2024-04-02at20 09 45-ezgif
com-video-to-gif-converter](https://github.com/elastic/kibana/assets/7893459/12ea4c85-9057-405c-9306-45c26ab27118)



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### Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to
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and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes |

|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces&mdash;unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes&mdash;Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
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| Code should gracefully handle cases when feature X or plugin Y are
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or plugin combination still results in our service operational. |
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packages/shared-ux/markdown/impl/markdown.tsx

@@ -87,6 +87,7 @@ export const Markdown = ({
     }
     return (
       <EuiMarkdownFormat
+        textSize={'relative'}
         className={className}
         aria-label={ariaLabelContent ?? 'markdown component'}
         parsingPluginList={_parsingPlugins}