Remove deprecated API fields, CLI flag and annotation

[shreyas-s-rao]

How to categorize this PR?

/area usability
/kind cleanup task

What this PR does / why we need it:
This PR removes the following deprecated elements in etcd-druid:

• API field Status.ServiceName - not useful from a consumer perspective
• API field Status.ClusterSize - use the length of Status.Members instead
• API field Status.UpdatedReplicas - use condition AllMembersUpdated instead, introduced in Introduce new Etcd status condition AllMembersUpdated #987
• API field Status.LastError - use Status.LastErrors instead, which allows population of multiple errors from the reconciliation flow
• CLI flag ignore-operation-annotation - use CLI flag enable-etcd-spec-auto-reconcile instead
• Annotation druid.gardener.cloud/ignore-reconciliation - use druid.gardener.cloud/suspend-etcd-spec-reconcile annotation instead

Please note that although field Status.LabelSelector has been marked as deprecated, it has not been removed, due to the fact that we might need to re-look at its use for vertical pod autoscaling via the /scale subresource.

Which issue(s) this PR fixes:
Fixes #801

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

Release note:

Remove deprecated status fields `ServiceName`, `ClusterSize`, `UpdatedReplicas` and `LastError`.

Remove deprecated CLI flag `ignore-operation-annotation`.

Remove support for deprecated annotation `druid.gardener.cloud/ignore-reconciliation`.

[unmarshall]

/lgtm