fix: user dedicated API to handle Group service account tokens

Jeremy Trequested to merge
jeremad/terraform-provider-gitlab:jrm/sa into main

Fix for: #6656

The terraform provider was not using the correct API for group service account tokens. This API was released later. We use this new API to avoid a bug in the Read function. The caveat is that the new API does not GET a single token, but rather all tokens associated with the given service account. That's why we have to loop over, to find the desired token.

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