Replace deprecated WithInsecure with WithTransportCredentials
gRPC has deprecated the WithInsecure option in favor of the more general WithTransportCredentials combined with insecure.NewCredentials. This commit removes our usage of the deprecated option so we don't get lint failures for using deprecated functionality when upgrading our gRPC dependency.
Failing job: https://gitlab.com/gitlab-renovate-forks/gitaly/-/jobs/2612937382
Dependency update: !4651 (merged)