Release v13.4.0

See merge request gitlab-org/gitlab-shell!402

Log SSH key details

See merge request gitlab-org/gitlab-shell!398

Right now when a client such as gitlab-shell calls the
`/api/v4/internal/allowed` API, the response only tells the client what
user has been granted access, and it's impossible to tell which deploy
key/token was used in the authentication request.

This commit adds logs for the following when available:

1. `gl_key_type` (e.g. `deploy_key` or `key`)
2. `gl_key_id`

These fields make it possible for admins to identify the exact record
that was used to authenticate the user.

API changes in the `/internal/allowed` endpoint in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/37289 are needed
to support this.

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/203

Log remote IP for executed commands

Closes #199

See merge request gitlab-org/gitlab-shell!399

Drop Go v1.12 support

See merge request gitlab-org/gitlab-shell!401

We officially use Go v1.13+ for everything
(https://docs.gitlab.com/ee/install/requirements.html#go-versions), and
dropping Go v1.12 allows us to use new methods such as
`http.NewRequestWithContext`.

Admins may want to know what client IP originated the request. This
commit adds a `remote_ip` field to the log that extracts the IP address
from the `SSH_CONNECTION` environment variable.

Closes https://gitlab.com/gitlab-org/gitlab-shell/-/issues/199

Support ssl_cert_dir config setting

See merge request gitlab-org/gitlab-shell!393