Update MCPConfig to use published ForgeFlower-Legacy

[AterAnimAvis]

See RetroGradle/MCPConfig#2 for current progress updating MCPConfig

Currently blocked on using official Forge Maven until all necessary components have been finalized and upstreamed

• MCInjector
• ForgeFlower

[TheCurle]

This is waiting on the pull and publishing of MinecraftForge/ForgeFlower#139.