Add possibility to remove AZs

[apjd]

/area control-plane
/area disaster-recovery
/area networking
/kind enhancement
/priority 3

What would you like to be added:

Possibility to remove an Availability Zone (AZ) from a shoot cluster

Why is this needed:

Entering wrong AZ configuration by mistake while setting up a new cluster must be repairable, e.g. entering the same name for "zone2" and "zone3" in the infrastructureConfig. In productive environments, removing and creating a new cluster to correct the AZ configuration may not be possible.

Example of an infrastructureConfig in a shoot.yml:

infrastructureConfig:
      apiVersion: aws.provider.extensions.gardener.cloud/v1alpha1
      kind: InfrastructureConfig
      networks:
        vpc:
          cidr: 10.250.0.0/16
        zones:
          - internal: 10.250.250.0/26
            name: ap-northeast-1a
            public: 10.250.246.0/26
            workers: 10.250.0.0/22
          - internal: 10.250.250.64/26
            name: ap-northeast-1c
            public: 10.250.246.64/26
            workers: 10.250.4.0/22
          - internal: 10.250.250.128/26
            name: ap-northeast-1c 
            public: 10.250.246.128/26
            workers: 10.250.8.0/22

To correct this infrastructureConfig, ap-northeast-1c must be replaced with ap-northeast-1d in one of the last zones.