Make Anthos version upgradeable

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By running terraform apply with an updated Anthos version, a user may expect this module to upgrade the installed Anthos version.

That is not the case today.

Doing so should upgrade the installed cluster.

terraform apply -var anthos_version=1.6.0 # cluster has been installed
terraform apply -var anthos_version=1.7.0 # cluster has been upgraded

[displague]

https://cloud.google.com/anthos/clusters/docs/bare-metal/1.6/how-to/upgrade

Seems the steps may be:

• ssh to the control plane nodes
• update the bmctl binary
• run bmctl upgrade cluster on the first control plane node (with the precise version)
• do something about the first user cluster?

We'll likely need to run the upgrade command in a separate null-resource provisioner block on every install, and handle the case where the command returns any kind of "already running that version" response.

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Upgrading may a require HA configuration.