DescribeBroker API error: unsupported format for Terraform

[pareshchalke]

When using Terraform with Moto Server to create an RabbitMQ broker, the JSON response for DescribeBroker contains CreationTimestamp in a format that Terraform does not accept. This causes Terraform to fail deserialization with the following error:

│ Error: reading MQ Broker (37680b): operation error mq: DescribeBroker, https response error StatusCode: 200, RequestID: Kcit2L20LGYDSFFaKQDqP2HkrSw4t5bNSitq3VkPLsERZZZwspwZ, deserialization failed, failed to decode response body with invalid JSON, expected __timestampIso8601 to be of type string, got json.Number instead

Moto returns CreationTimestamp in a format that Terraform does not accept, causing a deserialization failure.

I believe it requires similar changes - 216e586#diff-6b97e164285716cccad08ed0a1c43a0ba6becf99589b1881d91b9a4a2c47b907