fix: google_cloud_identity_group initialGroupConfig

[modular-magician]

According to 1 initialGroupConfig is an immutable parameter in both v1 and v1beta of the Cloud Identity groups API, and the updateMask for groups.patch does not accept it (it's a URL parameter in groups.create). Aside from the current implementation not correctly destroying and re-creating the resource when initial_group_config changes, requests in which no updateMask fields change results in a URL containing updateMask= which causes the API to error:

PATCH /v1/{name=groups/*}&updateMask=

{
"error": {
    "code": 400,
    "message": "Request contains an invalid argument.",
    "status": "INVALID_ARGUMENT"
}

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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Release Note Template for Downstream PRs (will be copied)

cloudidentity: fixed immutability issue on `initialGroupConfig` field for resource `google_cloud_identity_group`

Derived from GoogleCloudPlatform/magic-modules#7622