enable google_compute_instance_group to accept instances id

[edwardmedia]

Fixes hashicorp/terraform-provider-google#6526

Release Note Template for Downstream PRs (will be copied)

`compute`: enabled `google_compute_instance_group` `instances` to accept instance id field as well as self_link

[modular-magician]

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 19 insertions(+), 9 deletions(-))
Terraform Beta: Diff ( 3 files changed, 19 insertions(+), 9 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=119133"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 19 insertions(+), 9 deletions(-))
Terraform Beta: Diff ( 3 files changed, 19 insertions(+), 9 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=119134"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 3 files changed, 18 insertions(+), 8 deletions(-))
Terraform Beta: Diff ( 3 files changed, 18 insertions(+), 8 deletions(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=119135"