protobuf@3.6: disable (#105812)
protobuf@3.6: disable (#105812)

This has been deprecated for a while now. Let's disable it.

This would also potentially make room for a `protobuf@3` formula (with
version `3.20.x`) which seems like is needed given #105712.
adam-moss/homebrew-core,Homebrew/homebrew-core,adam-moss/homebrew-core,nbari/homebrew-core,chrisfinazzo/homebrew-core,filcab/homebrew-core,makigumo/homebrew-core,stgraber/homebrew-core,filcab/homebrew-core,nbari/homebrew-core,sjackman/homebrew-core,nandub/homebrew-core,nandub/homebrew-core,Homebrew/homebrew-core,stgraber/homebrew-core,sjackman/homebrew-core,lemzwerg/homebrew-core,lemzwerg/homebrew-core,chrisfinazzo/homebrew-core,zyedidia/homebrew-core,makigumo/homebrew-core,zyedidia/homebrew-core