Allow proper changing of google_compute_region_instance_group_manager.distribution_policy_target_shape

[askubis]

Allow proper changing of google_compute_region_instance_group_manager.distribution_policy_target_shape. Effectively - don't send the zones distribution when ONLY target shape changes.

fixes #19891

Release Note Template for Downstream PRs (will be copied)

compute: fixed an issue where immutable `distribution_zones` was incorrectly sent to the API when updating `distribution_policy_target_shape` in `google_compute_region_instance_group_manager` resource