Breaking change: Rework taint model in GKE

[modular-magician]

Part of #7928, #13309

This applies roughly the same model we're using with labels, where we limit what we write back into state during reads based on what's already in state. This gives us the opportunity to drastically simplify the interactions of the field, while unfortunately trading away authoritative-ness. We could easily reintroduce an authoritative field by making effective_taints O+C though- one of the benefits of effective_X output fields here and in the labels rework.

Not having update support is awkward- we'd be able to pick up new keys to manage if so, and delete keys, but that'll only update google_container_node_pool when added due to other complications with GKE cluster.

TestAccContainerNodePool_withSandboxConfig gets drastically simpler with this change- it's boring now! There's an unmanaged taint being added in the background there, with the withNodeConfig tests already covering taints added at create time.

Release Note Template for Downstream PRs (will be copied)

container: reworked the `taint` field in `google_container_cluster` and `google_container_node_pool` to only manage a subset of taint keys based on those already in state. Most existing resources are unaffected, unless they use `sandbox_config`- see upgrade guide for details.

container: added the `effective_taints` attribute to `google_container_cluster` and `google_container_node_pool`, outputting all known taint values

Derived from GoogleCloudPlatform/magic-modules#9011

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