Create an API to allow for auto-finalizing of upgrades

Currently, the Operator will by default set the preserve-downgrade option as this is the safest way to orchestrate an upgrade. For dev/test clusters, customers may want to not set this value. We should build an API that allows a customer to disable preserve-downgrade.

> @keith-mcclellan @alinadonisa
>
> > @taroface can we make sure that we have explicit upgrade instructions for the k8s operator that clarify that we always set the preserve downgrade option and the user is required to manually finalize the upgrade?
>
> The [current upgrade docs](https://www.cockroachlabs.com/docs/v20.2/orchestrate-cockroachdb-with-kubernetes.html#upgrade-the-cluster) instruct you to set `cluster.preserve_downgrade_option` and to manually finalize.
>
> Do you mean that:
>
> * The Operator automatically sets `cluster.preserve_downgrade_option` to the previous major version?

Yes, the operator does this automatically. The user just needs to finalize (we don't auto-finalize)

> * This means that there is never auto-finalization when using the Operator?

That is correct. We should backlog an issue to allow customers to auto-finalize for dev/test clusters.

_Originally posted by @keith-mcclellan in https://github.com/cockroachdb/cockroach-operator/issues/464#issuecomment-840569895_