Add field status.clusterSize and CEL rule for spec.replicas transitions

[shreyas-s-rao]

How to categorize this PR?

/area quality usability
/kind api-change enhancement

What this PR does / why we need it:
This PR adds a CEL rule to validate spec.replicas transitions, to ensure that transitions such as 3->1 and 1->0->3 are disallowed.

To make this work for hibernated clusters whose replicas is set to 0, a new field status.clusterSize to record the etcd cluster size, independent of the replicas. For instance, when the Etcd is scaled from 3 to 0 replicas, the cluster size still remains 3.

This PR also removes the test (test-unit) and test_integration steps from the concourse pipeline job, since these are now run as prow jobs and are stable.

Which issue(s) this PR fixes:
Fixes partially #1022

Special notes for your reviewer:
Integration tests and documentation changes will soon follow. Added integration tests and doc changes. PR is still on hold until druid v0.28.0 is released.
/hold

Also, concourse steps test and test_integration will fail since the executable scripts for these steps have been removed in this PR, but the pipeline definition is fetched from master branch, so the pipeline still attempts to run these steps. Once this PR is merged, subsequent PRs will not face this issue.

Release note:

Add CEL rule for validating `spec.replicas` transitions.

Add field `status.clusterSize` to record the etcd cluster size, independent of the `spec.replicas` value.

`test` and `test_integration` steps are removed from concourse pipeline job, since these are now run as [prow jobs](https://github.com/gardener/ci-infra/tree/master/config/jobs/etcd-druid).