Switch leader election to use Leases

[charith-elastic]

Since Kubernetes 1.14 a new leader election method based on the coordination.k8s.io API is available and it is the default method in controller-runtime since 0.7.0. We can switch to this new default once support for Kubernetes 1.13 is dropped from the operator.

kubernetes-sigs/controller-runtime#1144

[naemono]

In #5662 we moved to configmapleases as a temporary/interim measure. We released this in 2.3.0, and stayed on the 0.12.x branch of controller-runtime in 2.4.0. We should go ahead and moved to Leases and away from ConfigmapLeases in the next release.