In #18565 the tabletmanager gRPC service was updated to return vtrpcpb-style error codes

To achieve backwards compatibility with v22, the code was updated to support both the "new" and "old"/existing errors. In v24+ the "old" way should be removed

Places we need to update:
https://github.com/timvaillancourt/vitess/blob/main/go/vt/vttablet/tabletserver/throttle/base/metric_result.go#L75-L79

and

https://github.com/timvaillancourt/vitess/blob/main/go/vt/vttablet/tabletserver/throttle/throttler.go#L909-L917