Fix google_identity_group.

[modular-magician]

The enum has only one possible value and seems to be required in all cases.
This broke all existing terraform customers - and likely all API clients.
It has not been fixed in the autogenerated API tools, and it is not reflected in the docs.
I have reached out to the team to see if there is a possibility of fixing existing customers without an upgrade.

Fixes hashicorp/terraform-provider-google#7285.

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Fixed upstream breakage of `google_identity_group`.

Derived from GoogleCloudPlatform/magic-modules#3996