Fix `gitlab_service_slack` IDs

Patrick Decatrequested to merge
github/fork/pdecat/fix_service_slack_ids into main

Description

Resolves #1012 (closed)

Slack service IDs where changed in https://github.com/gitlabhq/terraform-provider-gitlab/pull/821#discussion_r842671210 which breaks all existing gitlab_service_slack resources which were created before v3.9.0 of the Gitlab terraform provider.

PR Checklist Acknowledgement

  • I acknowledge that all of the following items are true, where applicable:
    • Resource attributes match 1:1 the names and structure of the API resource in the GitLab API documentation.
    • Examples are updated with:
      • A *.tf file for the resource/s with at least one usage example
      • A *.sh file for the resource/s with an import example (if applicable)
    • New resources have at minimum a basic test with three steps:
      • Create the resource
      • Update the attributes
      • Import the resource
    • No new //lintignore comments were copied from existing code. (Linter rules are meant to be enforced on new code.)

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