Add concurrency limit for /gitaly.SSHService/SSHUploadPack to gitaly fleets

Incident: production#4944 (closed)

Add a 15 process concurrency limit to all gitaly nodes. It is possible that this could cause backlogs of long running transactions on the frontend SSHService. But we might better handle these types of loads in that area than we can on individual gitaly nodes.

Draft MR: https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/182