Bugfix gitlab_group_variable resource

The acceptance were passing with wrong provider resource, this commit uses testAccProtoV6MuxProviderFactories which is the correct provider for testing resources that depend on both SDK and framework versions.

The default value for gitlab_group_variable resource attributes was set as the acceptance test were failing since these values are expected to be set by provider and GitLab API, doesn't return default values causing provider state to drift.

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