resource/service_slack: changing the ID was a breaking change

GitLab Provider version

3.9.0 and above

GitLab version

GitLab Enterprise Edition 14.9.2-ee

Terraform version

1.1.7

Relevant Terraform Configuration

resource "gitlab_service_slack" "slack" {
  project = gitlab_project.git-project.id

  webhook  = var.service_integration_slack["webhook"]
  username = var.service_integration_slack["username"]

  push_events                = true
  issues_events              = true
  confidential_issues_events = true
  confidential_note_events   = true
  merge_requests_events      = true
  tag_push_events            = true
  note_events                = true
  pipeline_events            = true
  wiki_page_events           = true

  branches_to_be_notified = "all"

  confidential_issue_channel = lookup(var.service_integration_slack, "confidential_issue_channel", lookup(var.service_integration_slack, "channel", null))
  issue_channel              = lookup(var.service_integration_slack, "issue_channel", lookup(var.service_integration_slack, "channel", null))
  merge_request_channel      = lookup(var.service_integration_slack, "merge_request_channel", lookup(var.service_integration_slack, "channel", null))
  note_channel               = lookup(var.service_integration_slack, "note_channel", lookup(var.service_integration_slack, "channel", null))
  pipeline_channel           = lookup(var.service_integration_slack, "pipeline_channel", lookup(var.service_integration_slack, "channel", null))
  push_channel               = lookup(var.service_integration_slack, "push_channel", lookup(var.service_integration_slack, "channel", null))
  tag_push_channel           = lookup(var.service_integration_slack, "tag_push_channel", lookup(var.service_integration_slack, "channel", null))
  wiki_page_channel          = lookup(var.service_integration_slack, "wiki_page_channel", lookup(var.service_integration_slack, "channel", null))

  count = length(var.service_integration_slack) > 0 ? 1 : 0
}

Relevant log output

│ Error: GET https://myhost/api/v4/projects/2141/services/slack: 403 {message: 403 Forbidden}
│
│   with module.terraform.module.myproject.gitlab_service_slack.slack[0],
│   on .terraform/modules/terraform.myproject/gitlab-project.tf line 65, in resource "gitlab_service_slack" "slack":
│   65: resource "gitlab_service_slack" "slack" {
│

Description

Changing the ID of gitlab_service_slack resources from d.SetId(fmt.Sprintf("%d", service.ID)) to d.SetId(project) was a breaking change.

All existing gitlab_service_slack are now broken as their old ID is now considered the ID of their project, resulting in errors if you do not have the permissions to access the referenced project, e.g.:

│ Error: GET https://myhost/api/v4/projects/2141/services/slack: 403 {message: 403 Forbidden}
│
│   with module.terraform.module.myproject.gitlab_service_slack.slack[0],
│   on .terraform/modules/terraform.myproject/gitlab-project.tf line 65, in resource "gitlab_service_slack" "slack":
│   65: resource "gitlab_service_slack" "slack" {
│

Here, 2141 was the Slack service ID and is now considered a project ID.

And it probably has worse implications if you can access the related project. Originally posted by @pdecat in https://github.com/gitlabhq/terraform-provider-gitlab/pull/821#discussion_r842671210