Add changelog for 2025-06-19 (VapiAI#526)

Author: quantstruct-bot

fern/changelog/2025-06-19.mdx

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+# Workflow Configuration Enhancements and Assistant Updates
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+1. **Workflow-Level Configuration of Voice and Plans**: Developers can now configure `voice`, `transcriber`, and various plans like `monitorPlan` and `artifactPlan` at the workflow level. These configurations can still be overridden at the node level if needed. 
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+2. **Use of Dynamic Credentials in Workflows**: Workflows now support `credentials` and `credentialIds`, allowing you to specify dynamic credentials for workflow calls. This offers more flexibility in credential management, enabling credentials to be tied directly to specific workflows.
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+3. **`backgroundDenoisingEnabled` Deprecated in Assistants**: The `backgroundDenoisingEnabled` property in Assistant is now deprecated. You should use the new [`Assistant.backgroundSpeechDenoisingPlan`](https://api.vapi.ai/api#:~:text=SessionPaginatedResponse-,Assistant,-AssistantPaginatedResponse) to configure advanced background noise and speech denoising features.