[1.21.8] Fabric API 0.136.1+1.21.8

• Bump version (modmuss50)
• Backport FabricRenderState to 1.21.8 (#5004) (SkyNotTheLimit, EightSidedSquare)
• Bump version (modmuss50)
• Update AttachmentSync.java (#4921) (Fuzss)
• Implement an opt-in packet splitter. (#4841) (Patbox)