`DataPlane`: do not allow changes when a `DataPlane` in in the middle of a rollout

[pmalek]

Problem statement

To not interfere with the Blue Green rollout, we need to take into account the condition a DataPlane is in during admission.

Acceptance criteria

• [ ] The function validate is improved so that whenever a blue-green rollout is in progress, no change of the dataplane spec is allowed.
• [ ] The above change is tested to work with admission webhook (which uses the validator https://github.com/Kong/gateway-operator/blob/cdf64bc50676d42a5a951d62a796761255be253d/internal/admission/server.go#L63) and without it (https://github.com/Kong/gateway-operator/blob/971f1f473348bb83752e42d817380dcfcfcbe233/controllers/dataplane_controller.go#L94-L102). The latter is not yet enforced in BlueGreen controller.

[pmalek]

Punting this to Cloud Gateways Phase next.

Kong/gateway-operator-archive#1051 will cover the admission webhook part.