Update authentication diagram URL (#97320)
### What?
Updates the light and dark image references in the authentication guide
to use `authentication-overview-new.png`.
### Why?
The previous diagrams still showed `useFormState()` even though the
documentation has migrated to `useActionState()`.
Fixes #75709
### How?
Points both themed diagram references at the corrected replacement
assets.
Validation:
- Confirmed both replacement URLs return `200 image/png`
- `git diff --check`
