Use gcc v6.3 for CentOS 6 to build Rails C extensions
This will help unblock the upgrade to Ruby 2.7 since gRPC and google-protobuf need a more recent compiler.
As explained in https://stackoverflow.com/a/15602073/1992201:
The whole point of devtoolset is that it allows you to use a newer GCC and newer libstdc++ but without creating any run-time dependencies on the newer libstdc++ library. The compiled executables only need the system version of libstdc++.so that is always present on RHEL/CentOS, even systems without devtoolset installed. (What devtoolset does is package all the new libstdc++ features in a static library, called libstdc++_nonshared.a so that all the pieces not present in the system libstdc++.so get statically linked in, and everything else will come from the system libstdc++.so).
This will us move forward before dropping CentOS 6 support (#5163 (closed)).
Test build: https://dev.gitlab.org/gitlab/omnibus-gitlab/-/pipelines/163939