Convert Forge to use Official mappings.
Mojang released their obfuscation mappings but we have not used them up until now due to wanting to get their license to be more explicitly permissive.
It is clear that their intent is to allow us to use their names for developing mods like this.
See the full wording, and our interpretation here: https://github.com/MinecraftForge/MCPConfig/blob/master/Mojang.md
