Fix lease RBAC

[Wieneo]

How to categorize this PR?

/kind bug

What this PR does / why we need it:
This PR fixes a problem with the shoot-flux extension controller.
Currently the following errors get logged:

gardener-extension-shoot-flux-846946fc5b-b2l4x gardener-extension-shoot-flux {"level":"error","ts":"2025-06-24T11:10:21.415Z","msg":"Reconciler error","controller":"heartbeat","namespace":"","name":"","reconcileID":"c685e589-df4d-450b-a876-d926f8bc28a7","error":"leases.coordination.k8s.io \"gardener-extension-heartbeat\" is forbidden: User \"system:serviceaccount:extension-shoot-flux-l2tjc:gardener-extension-shoot-flux\" cannot patch resource \"leases\" in API group \"coordination.k8s.io\" in the namespace \"extension-shoot-flux-l2tjc\"","stacktrace":"sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).reconcileHandler\n\tsigs.k8s.io/[email protected]/pkg/internal/controller/controller.go:353\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).processNextWorkItem\n\tsigs.k8s.io/[email protected]/pkg/internal/controller/controller.go:300\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func2.1\n\tsigs.k8s.io/[email protected]/pkg/internal/controller/controller.go:202"}

Which issue(s) this PR fixes:
Fixes part of https://jira.schwarz/browse/STACKITSKE-3014