Scale down implementation does not work on juju agent >= 2.9.43

[zmraul]

With the release of this fix for juju: https://bugs.launchpad.net/juju/+bug/1951415 scaling down operations now don't immediately terminate the pod.

The current implementation of scale down on redis uses the instant failover (in case of primary scale down) that happened before the fix.

If redis is deployed on agent version 2.9.43, and a scale down operation is executed on the primary, redis server will elect a new primary, but sentinel doesn't get a proper SENTINEL RESET to relearn the status of the deployment.

[github-actions]

https://warthogs.atlassian.net/browse/DPE-2101