Update grpc-tools to match gRPC and google-protobuf versions

Following up on #2134 (closed),

We cannot include generated files in source packages in debian main (we have to generate it during build) but currently gitaly 1.59 fails to build with protobuf 3.7/grpc 1.19. So we were forced to update protobuf to 3.10/grpc 1.23 but which broke gitlab ssh access. Please use same versions of protobuf/grpc to help us maintain the debian package (we prefer to maintain only one version of protobuf/grpc). It'd be great to update protobuf 3.10/grpc 1.23 in both gitaly and gitlab.

_support/generate-proto-ruby
-> grpc_tools_ruby_protoc -I ./proto --ruby_out=ruby/proto/gitaly --grpc_out=ruby/proto/gitaly proto/blob.proto proto/cleanup.proto proto/commit.proto proto/conflicts.proto proto/diff.proto proto/namespace.proto proto/objectpool.proto proto/operations.proto proto/ref.proto proto/remote.proto proto/repository-service.proto proto/server.proto proto/shared.proto proto/smarthttp.proto proto/ssh.proto proto/storage.proto proto/wiki.proto
[libprotobuf WARNING google/protobuf/compiler/ruby/ruby_generator.cc:512] Omitting proto2 dependency 'google/protobuf/descriptor.proto' from proto3 output file 'shared_pb.rb' because we don't support proto2 and no proto2 types from that file are being used.
--ruby_out: shared.proto: Extensions are not yet supported for proto2 .proto files.
command failed: grpc_tools_ruby_protoc -I ./proto --ruby_out=ruby/proto/gitaly --grpc_out=ruby/proto/gitaly proto/blob.proto proto/cleanup.proto proto/commit.proto proto/conflicts.proto proto/diff.proto proto/namespace.proto proto/objectpool.proto proto/operations.proto proto/ref.proto proto/remote.proto proto/repository-service.proto proto/server.proto proto/shared.proto proto/smarthttp.proto proto/ssh.proto proto/storage.proto proto/wiki.proto

Update: Blocked by https://github.com/grpc/grpc/issues/21226 (waiting for new grpc release with embedded copy of protobuf updated to 3.11)