Update google-protobuf to 3.15.x to support Ruby 3.0

Why

Only labkit-ruby 0.17+ supports Ruby 3.0. However, labkit-ruby 0.17+ requires pg_query 2 (gitlab-org/labkit-ruby!62), which requires google-protobuf to 3.15.x.

Part of &5149 (closed)

Edited Apr 23, 2021 by Takuya Noguchi

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information