Clusters created via the dashboard set the `.spec.addons` field, even if no addons are selected. This will stop working with k8s 1.35

[voelzmo]

What would you like to be added:
Take care that

• clusters starting with k8s 1.35 no longer allow specifying any addons
• clusters created via the dashboard no longer set the .spec.addons field automatically if no addons are configured. This creates Shoot specs where owners need to do manual changes to, even when they never used addons at all

Why is this needed:
Currently all Shoots created via the dashboard get the .spec.addons field set, even if they don't use addons. This is also true for Shoots where the owners couldn't select any addons (because their purpose isn't evaluation). This creates Shoot specs which get deprecation warnings (starting from k8s 1.34) and will fail (starting from k8s 1.35).

Here is an example for a Shoot with purpose: development, which can't even use addons:

spec:
  addons:
    kubernetesDashboard:
      enabled: false
    nginxIngress:
      enabled: false
      externalTrafficPolicy: Cluster

See gardener/gardener#13845 for the changes in g/g