Switch to server-side-apply with Flux 2.8

[jfroy]

To migrate existing HRs:

kubectl get helmreleases -A -o jsonpath='{range .items[*]}{.metadata.namespace}{" "}{.metadata.name}{"\n"}{end}' | while read ns name
    kubectl patch helmrelease "$name" -n "$ns" --type=merge -p '{"spec":{"install":{"serverSideApply":true},"upgrade":{"serverSideApply":"enabled"},"rollback":{"serverSideApply":"enabled"}}}'
    kubectl -n "$ns" annotate --field-manager flux-client-side-apply --overwrite hr "$name" reconcile.fluxcd.io/requestedAt="$(date +%s)" reconcile.fluxcd.io/forceAt="$(date +%s)"
end