ci(milesonte): remove impracticable workflow

[grieve54706]

I follow a 2 years ago github action to use github issues API to update pull request. But it seems not work.
https://github.com/zoispag/action-assign-milestone/blob/master/src/milestone-actions.ts#L61

RequestError [HttpError]: Validation Failed: {"value":"0.12.2","resource":"Issue","field":"milestone","code":"invalid"}

{
  message: 'Validation Failed',
  errors: [Array],
  documentation_url: 'https://docs.github.com/rest/issues/issues#update-an-issue',
  status: '422'
}

I found the GitHub API document and I can not see the body can receive a milestone for updating pull requests.
https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#update-a-pull-request

Maybe auto-assigning a milestone to a new Pull request is impracticable. Let's give up.

[goldmedal]

so sad 😢