Silently inject task prompt as "AI Prompt" parameter

[johnstcn]

We are moving away from the "AI Prompt" Coder parameter to injecting the task prompt in the coder_ai_task data source instead.

This would be a breaking change for existing beta users of Coder tasks.

We can easily provide a workaround for this breaking change by automatically injecting the value of task.initial_prompt as the value of the "AI Prompt" parameter, if it is defined in the template.

This will give existing tasks users time to update their templates before we remove this behaviour.
Template admins should be shown some warning about the deprecation of "AI Prompt" as well so it doesn't just pass silently.

[johnstcn]

@DanielleMaywood showed that this is actually a no-op.