Reduce the max value of GCDrainStackTargetSize

`GCDrainStackTargetSize` is a flag used only by Parallel and G1 to control #entries on the stack for work-stealing. Depending on the particular value, it's possible that the cmdline value is silently ignored, which can be surprising.

A simple workaround is to shrink the accepted range -- leaving too many entries on the stack is undesirable.