fix: use h6 for message navigation #5605

[Aditya-Raj-25]

Fixes #5605

Description

This PR addresses the accessibility issue where navigating between messages was difficult for screen reader users.
It replaces the ScreenReaderText component with an <h6> element in ActivityRow.tsx.
The <h6> element is visually hidden but exposed to the accessibility tree, allowing users to navigate using the 'H' key.

Changes

• Modified packages/component/src/Transcript/ActivityRow.tsx to use <h6>.
• Added unit test packages/component/src/Transcript/ActivityRow.spec.tsx.

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[Aditya-Raj-25]

Please take a look at my pr @a-b-r-o-w-n

Thank you for your time.