`ApplySchema` command should reroute `ALTER VITESS_MIGRATION ... THROTTLE ...` via `UpdateThrottlerConfig`

[shlomi-noach]

In #13511 we routed vtgate ALTER VITESS_MIGRATION ... THROTTLE|UNTHROTTLE ... commenads away from the tablets and into topo via UpdateThrottlerConfig. The same was not done for vtctl ApplySchema, and tablet_executor.go still sends throttle/unthrottle commands to tablets. We should close that gap.