refactor(back): remove minimum bound on zone's maxVelocity

I set this randomly a while ago without thinking about it much, but game
code doesn't seem to limit this so don't know why backend should. If we
wanted to limit this, should be in zone validation logic. But someone
setting a <3500 maxvel on their map is such a weird case that I don't
think we should handle in code, can just bring up in review.

apps/backend/src/app/dto/map/map-zones.dto.ts

@@ -208,7 +208,7 @@ export class MapZonesDto /* extends JsonifiableDto */ implements MapZones {
     default: 3500
   })
   @IsNumber()
-  @Min(2000)
+  @Min(1)
   @Max(1000000000)
   @IsOptional()
   readonly maxVelocity?: number;