tasks: refactor: set column template_versions.has_ai_task when coder_task is referenced

[johnstcn]

Blocked by #977

A template version currently is defined as having an AI task if the Terraform graph has a coder_ai_task resource.

#977 introduces a coder_task data source. A template version should only be counted as having an AI task if it references this new data source.

Rationale: creating a coder_ai_task data source can be predicated on complex logic. template_versions.has_ai_task is primarily to restrict the possible template versions from which a Task could be created. Since the relevant information for a task will only be available in the coder_task data source, a template that does not reference this data source will not be able to create a Task.

[johnstcn]

We ended up keeping the resource instead.