Add changelog for December 21, 2024 (#122)

Co-authored-by: quantstruct[bot] <190039098+quantstruct[bot]@users.noreply.github.com>

Author: quantstruct[bot]

fern/changelog/2024-12-21.mdx

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+**Expanded Voice Compatibility with Realtime Models**: You can use the voices ash, ballad, coral, sage, and verse with any realtime models, giving you more flexibility in voice synthesis options.
+
+**Access to New OpenAI Models**:
+ You can now specify the new models `gpt-4o-realtime-preview-2024-12-17` and `gpt-4o-mini-realtime-preview-2024-12-17` when configuring `OpenAIModel.model` and `OpenAIModel.fallbackModels`.
+
+**New ElevenLabs Voice Models Available**:
+ The new voice models `eleven_flash_v2` and `eleven_flash_v2_5` are now available for use in `ElevenLabsVoice` and `FallbackElevenLabsVoice`, offering potential improvements in voice performance.