refactor(coder/agentapi): support terraform-provider-coder v2.12.0 (#485)

In terraform-provider-coder v2.12.0 and the
up-coming coder v2.28 release we have removed the
requirement for the "AI Prompt" parameter, and are intending on slightly
re-designing the API of the AI task modules.

Instead of `agentapi` defining the `coder_ai_task` resource, it will
output the `task_app_id`. Consumers of the module will then be expected
to create the `coder_ai_task` resource themselves with this
`task_app_id`.

registry/coder/modules/agentapi/README.md

@@ -16,7 +16,7 @@ The AgentAPI module is a building block for modules that need to run an AgentAPI
 ```tf
 module "agentapi" {
   source  = "registry.coder.com/coder/agentapi/coder"
-  version = "1.2.0"
+  version = "2.0.0"
 
   agent_id             = var.agent_id
   web_app_slug         = local.app_slug

registry/coder/modules/agentapi/main.tf

@@ -4,7 +4,7 @@ terraform {
   required_providers {
     coder = {
       source  = "coder/coder"
-      version = ">= 2.7"
+      version = ">= 2.12"
     }
   }
 }
@@ -239,8 +239,6 @@ resource "coder_app" "agentapi_cli" {
   group        = var.cli_app_group
 }
 
-resource "coder_ai_task" "agentapi" {
-  sidebar_app {
-    id = coder_app.agentapi_web.id
-  }
+output "task_app_id" {
+  value = coder_app.agentapi_web.id
 }