Changed crafting difficulty of some arrows

Changed Fabrication difficulty of Sharpened Metal Arrows, Heavy Fletched
Field Point Arrows, and Heavy Fletched Fire-Hardened Arrows from 5 to 4.

Previously, all components for heavy arrows were level 3, while all
heavy and metal arrows were level five, creating a gap in leveling
progress.   This update allows the improvised point heavy arrows to
serve as fabrication skill stepping stone.  Now archers don't have to
spend level 4 pointlessly grinding through unrelated recipes, but the
vastly superior Heavy Wooden and Metal arrows are still sought after.
Merge pull request #9364 from Skidborg/arrow-crafting-progression

Fletching Difficulty Tweak