Allow updates to controlPlaneEndpoint

[varshavaradarajan]

For restore operations, it can become necessary to provision a new control plane droplet or loadbalancer. Currently, the docluster webhook prevents updates to docluster.spec.ControlPlaneEndpoint if it is not empty. We need to remove this validation to allow restores

Slack refs:
https://kubernetes.slack.com/archives/CTND9FHU2/p1644935655155549?thread_ts=1644920088.841939&cid=CTND9FHU2
https://kubernetes.slack.com/archives/CTND9FHU2/p1639149619033900?thread_ts=1639075525.031700&cid=CTND9FHU2

@cpanato - would it be possible to backport this fix to v0.5.x?

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/lifecycle stale

[timoreimann]

/remove-lifecycle stale