contrib: update protobuf/protoc to 3.21.1

It needs posix thread model on Windows builds (ref. #28498).

The 3.21 branch is the last one that doesn't depend on abseil-cpp which is not even included in the source. It's a big C++ library that installs a lot of things and takes a long time to build.

Higher 3.21.x versions have an issue with memory alignment on armv7 with Clang older than 16.