Update Gitaly module dependency

This updates Gitaly's latest Protocol Buffer definitions, which were last updated in Feburary 2020, with the latest version of Gitaly. This is done to ensure the definitions are in sync with Workhorse (gitlab-workhorse!619 (merged)) and Gitaly.

Because we started tagging Gitaly versions v13.x.x instead of v1.x.x, go get refuses to update the Gitaly module using the latest tag or SHA (gitaly#3177 (closed)). Go tries to enforce semantic compatibility by requiring the use of module paths (e.g. m/v13), and Gitaly doesn't adhere to this scheme.

To workaround this issue, we use the replace directive (https://github.com/golang/go/wiki/Modules#when-should-i-use-the-replace-directive):

go mod edit -replace gitlab.com/gitlab-org/gitaly=gitlab.com/gitlab-org/gitaly@3f5e218def93024f3aafe590c22cd1b29f744105

gitlab-shell appears to uses the following messages:

1. SSHReceivePackRequest
2. SSHUploadArchiveRequest
3. SSHUploadPackRequest

gitlab-workhorse!619 (comment 421789354) shows that none of these RPCs were changed.