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.

Make it possible to propagate correlation ID across processes

Closes #474

See merge request gitlab-org/gitlab-shell!413

Previously, gitlab-shell did not pass a context through the application.
Correlation IDs were generated down the call stack instead of passed
around from the start execution.

This has several potential downsides:

1. It's easier for programming mistakes to be made in future that lead
to multiple correlation IDs being generated for a single request.
2. Correlation IDs cannot be passed in from upstream requests
3. Other advantages of context passing, such as distributed tracing is
not possible.

This commit changes the behavior:

1. Extract the correlation ID from the environment at the start of
the application.
2. If no correlation ID exists, generate a random one.
3. Pass the correlation ID to the GitLabNet API requests.

This change also enables other clients of GitLabNet (e.g. Gitaly) to
pass along the correlation ID in the internal API requests
(https://gitlab.com/gitlab-org/gitaly/-/issues/2725).

Fixes https://gitlab.com/gitlab-org/gitlab-shell/-/issues/474

hooks: Remove deprecated hooks dir

See merge request gitlab-org/gitlab-shell!411

Over a year ago, through e0824f17367a44e5bac84dac7197f9b45e37ab01, the
hooks in this repository were updated to not be successful, basically
block each push from happening.

This change removes the hooks, and updates the documentation. There's no
customer impact, and these changes are just part of a general cleanup.