Update release drafter template and config

[jiholee17]

Add template for release-drafter, and disable release-drafter for PRs. update_release_draft Github Action for PRs has been succeeding but failing silently in the logs, and now fails loudly in release-drafter@v7:

...
Error: Validation Failed: {"resource":"Release","code":"invalid","field":"target_commitish"}
{
  name: 'HttpError',
  id: '21607234714',
  status: 422,
  response: {
    url: 'https://api.github.com/repos/Netflix/dgs-codegen/releases/267842614',
    status: 422,
    headers: {
...

and release drafter isn't applicable for PRs as noted in releasedraft.yml:

  # pull_request event is required only for autolabeler