Execute an available update against a cluster that is a registered component of the specified
system, identified by the update definition id in the path.

The update runs asynchronously: this endpoint returns 202 Accepted with a pending
update operation, and the update then transitions through pending → running → completed/failed
via a background job. This system-scoped endpoint set has no poll endpoint — to poll the
resulting cluster update execution status, use the existing GET /api/clusters/{id}/updates/{updateId}
endpoint (Clusters tag).

See also the non-system-scoped POST /api/clusters/{id}/execute-update (Clusters tag), which
executes the same update for a cluster without requiring system membership. This system-scoped
variant additionally requires the cluster to be a registered component of the named system
(else 404), and the update definition must be applicable to the cluster's type (else 400). There
is no request body.

Requires the infrastructure-system permission with full access.