[Remote Store] Validate that remote cluster state is enabled while setting migration direction to remote-store

[astute-decipher]

Is your feature request related to a problem? Please describe

We want to disallow migration direction to be set remote-store in mixed compatibility mode case remote cluster state is not already enabled. This is to ensure that we garbage collect the stale data uploaded to s3 while migrating and we fallback to DocRep due to some failure.

Describe the solution you'd like

We will add a validation in TransportClusterUpdateSettingsAction to check if cluster.remote_store.state.enabled is true in cluster settings if the update setting request is trying to set migration direction to remote-store and cluster is either already in mixed mode or being requested along with.

Related component

Storage:Remote

Describe alternatives you've considered

No response

Additional context

No response

[astute-decipher]

Not required now.