Add FAQ about ElevenLabs model compatibility

Add new FAQ entry clarifying that replacement voices work with any
ElevenLabs models out of the box.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Vapi Tasker

fern/providers/voice/vapi-voices/legacy-migration.mdx

@@ -105,4 +105,8 @@ For voices that are not in the list of voices on the Vapi Dashboard, users will
   <Accordion title="Are the staying voices (Elliot, Savannah, Leo, etc.) affected?">
     No. Elliot, Savannah, Leo, Zoe, Mia, Jess, Zac, Dan, Leah, Tara, and Rohan are continuing and are not being retired. If you're using one of these voices, no action is required.
   </Accordion>
+
+  <Accordion title="Are these voices compatible with specific ElevenLabs models?">
+    Replacement voices should work out of the box with any of the ElevenLabs models.
+  </Accordion>
 </AccordionGroup>