Prepare removal of guest_accelerator ConfigModeAttr

[modular-magician]

part of #12824

guest_accelerator's ConfigModeAttr can be directly removed in 6.0 because:

1. existing logic to make guest_accelerator.count = 0 count as an empty entry, therefore an alternative way to send an empty list
2. The field is ForceNew, meaning we don't need update functionality

I've manually tested that there is no difference between count = 0 and guest_accelerator = [] for google_container_cluster, google_container_node_pool, and google_compute_instance, and have added tests for them as well.

The child fields guest_accelerator.gpu_driver_installation_config and guest_accelerator.gpu_sharing_config have been added AFTER the 0.11->0.12 upgrade and look to have just copied the ConfigModeAttr attribute mistakenly: https://github.com/GoogleCloudPlatform/magic-modules/pull/8348/files#diff-058ffca5a3cb3663c791422e055902e433e8f27f44fd144c667cfe5ed95556eeR117 and https://github.com/GoogleCloudPlatform/magic-modules/pull/6628/files#diff-542bf08ccbb9e20a2abbf127fde6d38f61c83124b05370a5e53d3e4caa3512f4R110
In the rare chance a user has somehow utilized the empty block configuration [] for these, just removing the fields entirely will not produce a diff due to O+C.

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#11425

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