Testify features sub packages `assert` and `require`. The difference is
subtle, and lost on novice Golang developers that don't read the docs.
To create a more consistent code base `assert` will no longer be used.

This change was generated by a running a sed command on all `_test.go`
files, followed by `goimports -w`.

The user agent for requests to the internal API endpoints used the
default Go provided user agent. This change updates that to always set
something else, by default `GitLab-Shell`.

Than for others importing the package, there's a new API to set it to
something else. This has been done with new method, a setter, to
maintain backwards compatibility in the API.

Drop "generated random correlation ID" log message

See merge request gitlab-org/gitlab-shell!417

This message happens all the time and doesn't add a lot of value.

Relates to https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/1275

Release v13.8.0

See merge request gitlab-org/gitlab-shell!416

Update Gitaly module dependency

See merge request gitlab-org/gitlab-shell!414

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
(https://gitlab.com/gitlab-org/gitlab-workhorse/-/merge_requests/619)
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 (https://gitlab.com/gitlab-org/gitaly/-/issues/3177).  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
```

config: Set a secret example

See merge request gitlab-org/gitlab-shell!415

The config.yml.example didn't include a field I was expecting to be
there, which lead me to believe the field didn't exist. This change adds
the `secret` YAML field, and describes how it interacts with the
secrets_file.