[Feature] Prepare Druid for Server-Side Apply

[timuthy]

Feature (What you would like to be added):
Druid should use and support Server-Side Apply where applicable once Gardener dropped the support for seed clusters with K8s <= 1.17 (gardener/gardener#4083).

Motivation (Why is this needed?):
Server-Side Apply makes working with the etcd resource more efficiently when there will be more then one actor (motivated here).

Tasks to be done:

• Add markers for merge strategy to etcd resource
• Use SSA for etcd status updates

[timuthy]

We experienced some major issues with SSA in our controllers:

• x-kubernetes-list-map-keys does not work properly with any version < 1.21 (for CRDs) which makes it impossible to use SSA for arrays, e.g. status.conditions.
• Further issues are mentioned in gardener/gardener#4122

Therefore, we close this issue and will work with alternatives (like #206) to avoid conflict errors.