Remove `newly_detected` in %17.0

Description

Based on &6826 (comment 1341377224), we determined:

1. New
   1. Needs Triage
   2. Dismissed
2. Previously Existing
   1. Needs Triage
   2. Confirmed
   3. Dismissed
   4. Resolved

The current values map to this as follows:

• newly_detected -> Both New::Needs Triage and New::Dismissed
• detected -> Previously Existing::Needs Triage
• confirmed -> Previously Existing::Confirmed
• dismissed -> Previously Existing::Dismissed
• resolved -> Previously Existing::Resolved

I recommend we continue to support newly_detected (to avoid a breaking change) and then also add support for two new values that map New::Needs Triage and New::Dismissed. We could potentially deprecate support for newly_detected and remove it in %17.0.

This issue will capture removal of the newly_detected field.