Add support for PDB maxUnavailable in EnvoyProxy CRD

[Skaronator]

Description: The EnvoyProxy allows to configure a podDisruptionBudget. Currently that CRD allows to only configure a minAvailable but it would be way more useful to also support maxUnavailable.

[optional Relevant Links:]

maxUnavailable is also recommended by the official Kubernetes docs:

The use of maxUnavailable is recommended as it automatically responds to changes in the number of replicas of the corresponding controller.

https://kubernetes.io/docs/tasks/run-application/configure-pdb/#:~:text=The%20use%20of%20maxUnavailable%20is%20recommended%20as%20it%20automatically%20responds%20to%20changes%20in%20the%20number%20of%20replicas%20of%20the%20corresponding%20controller.

Original PR: #3583

Kubernetes PDB: Why we swapped to using maxUnavailable