fix: Standardize ElevenLabs auto-configuration conditional property

[apappascs]

Update ElevenLabsAutoConfiguration to use the standardized SpringAIModelProperties AUDIO_SPEECH_MODEL property instead of the provider-specific enabled property.

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[apappascs]

@ilayaperumalg the ElevenLabs auto-configuration wasn't updated to use the unified model selection property like OpenAI TTS and all the chat models do!