Reconcile `/clear`, `/clean`, `/reset` slash commands in Duo Chat; replace all with `/new`

Context

This issue is a follow-up from discussions #468439 (closed)

Problem to solve

We added slash commands to Duo Chat over time to address different needs. Some of the commands added were addressing GitLab staff needs, rather than user needs - e.g. /clear was added to make it easier for GitLab staff to facilitate demos.

The overall guidance on slash commands to have a small, curated list of the most important commands. Each command should be easy to understand, without needed to read the subtitle text.

The difference between the words /clear, /clean, and reset is ambiguous.

Also, at the time of writing:

• The chat history auto-deletes after 72 hours, so /reset only preserves the history temporarily
• We do not have concrete evidence that starting a new conversation or clearing the history produces better results
• We do not know how common it is for users to refer to their history

Suggested solution

• Replace /clear, /clean, /reset slash commands with a single command: /new
• /new functions the same as /clear
• /reset functions the same as /clear if and only if the duo_chat_multi_thread feature flag is on (which is isMultithreaded in duo-ui), otherwise it behaves as current behavior.
• We still support /clear, /clean, /reset (technically, under-the-hood) but they do not appear in the slash command UI menu
• Actionable insight: users who already use /clean and /reset were confused by /new in user testing. Make sure any solution has an onboarding state to inform users of the change

Rationale

• We show a small list of default slash commands in the menu UI, so users have less to parse and remember
• The words new are more explicit and clear what the functionality is than reset, clear etc.
• We still support the older iterations of this command (without a UI, and except /reset) for GitLab staff who need to make demos, and for users who have already learned these commands