fix: [Stateful: Indices Overview page] Inaccurate announcement for programming language buttons (#197428)

Closes: #196282

## Summary

Button elements should be clear and understandable to users, including
those using assistive technologies. `h5` tag looks unnatural inside a
button.

## What was changed?: 

1. `h5` -> `strong`

## Screen: 

<img width="1190" alt="image"
src="https://github.com/user-attachments/assets/ea02864f-4091-4cc9-a674-ebf1f4ee7499">

packages/kbn-search-api-panels/components/language_client_panel.tsx

@@ -62,8 +62,12 @@ export const LanguageClientPanel: React.FC<SelectClientProps> = ({
                 width={euiTheme.size.xl}
               />
               <EuiSpacer size="s" />
-              <EuiText textAlign="center" color={isSelectedLanguage ? 'default' : 'subdued'}>
-                <h5>{language.name}</h5>
+              <EuiText
+                size="relative"
+                textAlign="center"
+                color={isSelectedLanguage ? 'default' : 'subdued'}
+              >
+                <strong>{language.name}</strong>
               </EuiText>
             </EuiFlexItem>
           </EuiFlexGroup>