Update to 1.21

Added mods:

• 3D Skin Layers
• Better Statistics Screen
• CalcMod
• Chat Heads
• Dark Loading Screen
• Mouse Tweaks (because Mouse Wheelie has no 1.21 version)

Mods I can't add because they are not updated to 1.21 and have no alternative:

• Shulker Box Tooltip
• CIT Resewn
• WaveyCapes

Temporary replacement mods:

• Mouse Wheelie -> Mouse Tweaks
• Cubes Without Borders -> Concentration