Revert update google-protobuf to 3.15.8

In !3399 (merged) the google-protobuf gem was updated to 3.15.8.

But since then, segfaults are reported in CI on gitlab-org/gitlab:

• https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/1233313735
• https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/1234044791

This change reverts the upgrade until we know more about the stability of v3.15.8.