docs: update Vapi Voices page with new voices and retired banner

[betzlermeow]

Description

• Updated the legacy voices banner to reflect that the voices were recently retired (past tense) instead of being retired on a future date
• Updated Elliot and Savannah voice characteristics and age format (from "25 years old" to "20s")
• Added 8 new voice listings: Rohan, Emma, Clara, Nico, Kai, Sagar, Godfrey, and Neil
• Added "New" tags for newly introduced voices: Emma, Clara, Nico, Kai, Sagar, Godfrey, and Neil
• Rohan is an existing voice newly listed on this page (no "New" tag)
• Only Elliot and Savannah retain audio sample previews; no previews for new voices
• Total voices on page: 10 (up from 2)

Resolves DEVREL-522

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work
• Verify that all 10 voices appear in the correct order: Elliot, Savannah, Rohan, Emma, Clara, Nico, Kai, Sagar, Godfrey, Neil
• Verify the legacy voices banner displays the updated past-tense text
• Verify "New" tags appear for Emma, Clara, Nico, Kai, Sagar, Godfrey, and Neil
• Verify Rohan does NOT have a "New" tag
• Verify audio samples only appear for Elliot and Savannah
• Verify the "Learn more" link still points to the legacy migration page

[github-actions]

🌿 Preview your docs: https://vapi-preview-caeec9ec-d060-4587-a0f0-d4335dda29fc.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs: https://vapi-preview-80939f18-74e3-4696-a2e2-71990700e152.docs.buildwithfern.com