Fix get_commit_signatures feature flag

Some specs are failing because of get_commit_signatures feature flag:

https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/271054310

The feature behind the flag is already on Gitaly, so we need to add the flag again and fix the specs.

The following discussion from !31743 (merged) should be addressed:

• @stanhu started a discussion: (+1 comment)

  FYI @felipe_artur and @johncai. The specs were failing in https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/271054310.