adopt SchemaVersion and StateUpgraders where relevant

[mdb]

It would be useful to consider the use of schema.Resource.SchemaVersion on terraform-provider-vinyldns resources such that schema.Resource.StateUpgraders could be leveraged to upgrade existing users' tfstate when schema semantics change such that users can easily adopt new versions of the provider.

More can be read here:

    // StateUpgraders contains the functions responsible for upgrading an
    // existing state with an old schema version to a newer schema. It is
    // called specifically by Terraform when the stored schema version is less
    // than the current SchemaVersion of the Resource.
    //
    // StateUpgraders map specific schema versions to a StateUpgrader
    // function. The registered versions are expected to be ordered,
    // consecutive values. The initial value may be greater than 0 to account
    // for legacy schemas that weren't recorded and can be handled by
    // MigrateState.
    StateUpgraders []StateUpgrader