Update protobuf libraries

[mpwarres]

As pointed out in envoyproxy/envoy#29546, proxy-wasm-cpp-sdk is using v3.9.1 of the protobuf library, which is from 2019. We should update it.

[martijneken]

+1 we should do this. I tried as part of my emsdk update (to fix a zlib issue) and I ran into some issues propagating copts: #157 (comment)

I propose we finish the emsdk update first, then try to bump protobuf.

[mpwarres]

Updating the protobuf library should make the hack mentioned in these instructions obsolete, since protobuf added support for building with Emscripten in protocolbuffers/protobuf@8e869f7.

As part of updating protobuf, the instructions linked above should be removed.