Start using new confirm_start_over_before_letter route

[soniaconnolly]

🎫 Ticket

LG-11126 followup

🛠 Summary of changes

Created a new route in #9313. Once that's merged, we can use it here.

(This PR is really only the final two commits, the rest are from the previous PR and are already on main.)

📜 Testing Plan

• Create account, start IdV
• At phone step, choose Verify by mail
• On RequestLetter ("Verify your Address") page, click cancel and start over. It should look like the "New" screenshot in LG-11126 Update Start over verifying your identity screen #9313 , including "To mail your letter to another address"
• Continue through to Account page and then Enter Code ("Welcome back") page
• Click cancel and start over. It should look like the "Existing" screenshot in LG-11126 Update Start over verifying your identity screen #9313 , including "The verification code in your letter will no longer work."