Fix version behaviour in google_compute_instance_group

[rileykarson]

Fixes TestAccComputeInstanceGroup_network, TestAccComputeInstanceGroup_update

Followup to #2483

Release Note Template for Downstream PRs (will be copied)

`compute`: fixed diffs in `google_compute_instance_group`'s `network` field when equivalent values were specified

`compute`: fixed issues updating `google_compute_instance_group`'s `instances` field when config/state values didn't match

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, e2f1f22.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#1286
depends: hashicorp/terraform-provider-google#4728

[rileykarson]

@megan07: Got another test that was failing, and the fix ended up touching a different part of the resource. Mind taking another look?

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 63db0bb.

Pull request statuses

terraform-provider-google-beta already has an open PR.
terraform-provider-google already has an open PR.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: GoogleCloudPlatform/terraform-google-conversion#235