docs: Add steps to remove v1 deployment

Signed-off-by: Moritz Wanzenböck <[email protected]>

docs/how-to/upgrade/3-remove-operator-v1.md

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+# Remove the Piraeus Operator v1 Deployment
+
+Migrating to the new Piraeus Operator deployment requires temporarily removing the existing deployment.
+
+After this step no new volumes can be created and no existing volume can be attached or detached, until the
+new deployment is rolled out.
+
+This is the third step when migrating Piraeus Operator from version 1 (v1) to version 2 (v2).
+[Click here to get back to the overview](./index.md).
+
+## Prerequisites
+
+* [Install `kubectl`](https://kubernetes.io/docs/tasks/tools/)
+* [Install `helm`](https://docs.helm.sh/docs/intro/install/)
+
+## Scale Down the Operator Deployment
+
+To prevent modification of the existing cluster, scale down the existing Piraeus Operator deployment.
+
+```
+$ helm upgrade piraeus-op ./charts/piraeus --set operator.replicas=0
+$ kubectl rollout status -w deploy/piraeus-op-operator
+deployment "piraeus-op-operator" successfully rolled out
+```
+
+## Remove the Finalizers from the Piraeus Resources
+
+The Operator sets Finalizers on the resource it controls. This prevents the deletion of these resources when the
+Operator is not running. Remove the Finalizers by applying a patch:
+
+```
+$ kubectl patch linstorsatellitesets piraeus-op-ns --type merge --patch '{"metadata": {"finalizers": []}}'
+linstorsatelliteset.piraeus.linbit.com/piraeus-op-ns patched
+$ kubectl patch linstorcontrollers piraeus-op-cs --type merge --patch '{"metadata": {"finalizers": []}}'
+linstorcontroller.piraeus.linbit.com/piraeus-op-cs patched
+```
+
+## Remove the Piraeus Resources
+
+Having removed the Finalizers, you can delete the Piraeus Resources. This will stop the LINSTOR Cluster, and no
+new volumes can be created, and existing volumes will not attach or detach. Volumes already attached to a Pod will
+continue to replicate.
+
+```
+$ kubectl delete linstorcsidrivers/piraeus-op
+$ kubectl delete linstorsatellitesets/piraeus-op-ns
+$ kubectl delete linstorcontrollers/piraeus-op-cs
+```
+
+## Remove the Piraeus Deployment
+
+As a last step, you can completely remove the helm deployment, deleting additional resources such as service accounts
+and RBAC resources. In addition, also clean up the Custom Resource Definitions:
+
+```
+$ helm uninstall piraeus-op
+$ kubectl delete crds linstorcontrollers.piraeus.linbit.com linstorsatellitesets.piraeus.linbit.com linstorcontrollers.piraeus.linbit.com
+```
+
+## Optional: Remove additional Piraeus Components
+
+If you have deployed additional components from Piraeus, such as the HA Controller or LINSTOR Affinity Controller, you
+will need to remove them, too. After completing the migration, you can install them again, if needed.
+
+```
+$ helm uninstall piraeus-ha-controller
+$ helm uninstall linstor-affinity-controller
+```