chore: tasks: replace exising usage of hard-coded "AI Prompt" parameter with `tasks.prompt`

[johnstcn]

Blocked by #948

Tasks currently have a hard-coded dependency on a parameter named AI Prompt (defined as TaskPromptParameterName in coder/terraform-provider-coder.
This is to be replaced with tasks.prompt.

• Deprecate both codersdk.TaskPromptParameterName and provider.TaskPromptParameterName
• Replace all existing references to this parameter name with tasks.prompt WHERE tasks.id = ?