How to categorize this PR?

/area control-plane monitoring ops-productivity usability
/kind enhancement api-change

What this PR does / why we need it:
This PR introduces a new Etcd status condition AllMembersUpdated, to indicate whether all the underlying etcd member pods have been updated with the latest Etcd spec changes after reconciliation. This allows consumers of the Etcd status to know when a rollout of their spec changes has completed.

Additionally, condition statuses Progressing and ConditionCheckError are now deprecated, and will soon be removed, since druid conditions will be replaced by metav1.Condition in the future.

Also, a small change has been made to the etcd components webhook handler, to output the druid.gardener.cloud/disable-etcd-component-protection annotation when disallowing an operation on an etcd component managed by druid. This helps users avoid searching through the documentation for the specific annotation to add in order to disable the webhook check.

Which issue(s) this PR fixes:
Fixes #985

Special notes for your reviewer:
/invite @unmarshall
/assign @unmarshall
/cc @timuthy

Release note:

Introduce new Etcd status condition `AllMembersUpdated`.

Condition statuses `Progressing` and `ConditionCheckError` are now deprecated and will soon be removed, since druid conditions will be replaced by `metav1.Condition` in the future.