Port mod to Minecraft 1.20.1

[ciefa]

Major changes:

• Downgrade Minecraft from 1.21.1 to 1.20.1
• Switch from NeoForge to Forge mod loader
• Downgrade Java from 21 to 17
• Update Gradle wrapper from 9.x to 8.1.1

Build system changes:

• Replace net.neoforged.moddev plugin with ForgeGradle 6.x
• Add Parchment mappings support for Forge
• Add Mixin plugin for SpongePowered Mixin support
• Update mods.toml format for Forge (moved to META-INF)

API changes:

• Update all imports from net.neoforged.* to net.minecraftforge.*
• Use @SubscribeEvent instead of addListener for events
• Update TickEvent.ClientTickEvent handling with phase check
• Update GuiMixin for 1.20.1 render signature (float partialTick instead of DeltaTracker)
• Update mouseScrolled signature (3 params instead of 4)
• Update renderBackground signature (no extra params)

Other updates:

• Update mixin compatibility level to JAVA_17
• Update README for Forge/1.20.1
• Update GitHub workflow to use Java 17