modules/gcp_compute_instance_group_info.py doesn't return "instances"

Current implementation if https://github.com/ansible-collections/google.cloud/blob/master/plugins/modules/gcp_compute_instance_group_info.py mentions return parameter:

```
instances:
description:
- The list of instances associated with this InstanceGroup.
- All instances must be created before being added to an InstanceGroup.
- All instances not in this list will be removed from the InstanceGroup and
will not be deleted.
- Only the full identifier of the instance will be returned.
returned: success
type: list
```

but there is no calls to ` POST /compute/v1/projects/{project}/zones/{zone}/instanceGroups/{resourceId}/listInstances`.

See https://cloud.google.com/compute/docs/reference/rest/v1#rest-resource:-v1.instancegroups

The only file with a call to `.../instanceGroups/{resourceId}/listInstances` is `gcp_compute_instance_group.py` and it modify state...

Start with plugins/modules/gcp_compute_instance_group_info.py and compare its implementation with the listInstances call in gcp_compute_instance_group.py. Check the Google Compute Engine instanceGroups listInstances REST endpoint and confirm that successful module output includes the documented instances list without modifying state.

Written by the indexing model from the issue text.