Update placeholder text in chat input

Context

There is a placeholder text in the chat input that says You can ask the AI for more information.

Problem to solve

• This placeholder is shown if you haven't asked anything at all (what is more referring to?)
• The placeholder has a full stop . at the end. Our placeholders don't typically have that
• The placeholder is not a strong call to action
• We don't call it "the AI" - it's GitLab Duo Chat

Suggestion

Update the placeholder to:

• Ask GitLab Duo