Introduce an immutable value to track (and lock) if cluster control plane is HA or not

This is a follow-up of #667 (closed) and !1018 (comment 1674038017)

We should introduce a value to track if cluster is HA or not and prevent users from switching from non-HA to HA.

This variables should be used in various places to compute pods replicas and disruption budgets

We should prevent users to move from non-HA to HA, as they would feel to have an HA cluster whereas some critical services like vault would remain single-headed.