fix(kafka/cluster): clusterpolicy update + delete

[wotolom]

Description of your changes

For kafka.Cluster:

• adaptation in getClusterPolicyState to allow triggering deletion of ClusterPolicy when user sets clusterPolicy: null in claim
• cache the CurrentVersion of the ClusterPolicy to enable making consecutive PutClusterPolicy calls
  EDIT: add + use new field Status.AtProvider.ClusterPolicyVersion to "cache" the CurrentVersion of the ClusterPolicy for update. This way user has also access to that data, e.g. for making manual PutClusterPolicy calls.

Fixes #1920

I have:

• Read and followed Crossplane's [contribution process].
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

manually

[MisterMX]

LGTM. Thank you very much @wotolom!

[github-actions]

Successfully created backport PR #1922 for release-0.44.