Add aria-label for Duo chat prompt

What does this MR do and why?

While reviewing a recent change to Duo chat, I realised there's an aria-label missing on the submit button. There is already a string for, but used only in Pipeline Editor chat. So I'm reusing it for Duo as well.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Screen Recording 2023-09-19 at 18.06.44.mov	Screen Recording 2023-09-19 at 18.04.43.mov

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Setup Due locally Once you have the API key, I found this script very helpful: $2554994
2. Go to the sidebar, click Help, then Ask GitLab Duo. With a screen reader working, submit a question to the chat.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.