Use constants for a few magic numbers in rules

src/main/java/carpet/mixins/StructureBlockEntity_fillUpdatesMixin.java

@@ -6,7 +6,6 @@ import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Redirect;
 
-import java.util.Random;
 import net.minecraft.core.BlockPos;
 import net.minecraft.world.level.ServerLevelAccessor;
 import net.minecraft.world.level.block.entity.StructureBlockEntity;