Currently, we perform a request to Gitlab Rails that proxies
the request to primary

However, it causes timeouts on big pushes and consumes large
amount of memory. We can perform an HTTP request directly
from Gitlab Shell instead and stream the response to the user

This commit also excludes gitlab-shell from dependencies:

Gitaly specifies Gitlab Shell as a dependency as well in order
to use gitlabnet client to perform API endpoints to Gitlab Rails.
As a result, Gitlab Shell requires Gitaly -> Gitaly requires an
older version of Gitlab Shell -> that version requires an older
version of Gitlab Shell, etc. Let's use exclude to break the
chain earlier

While gitlab-shell currently has a major version of v14, the module path
it exposes is not using that major version like it is required by the Go
standard. This makes it impossible for dependents to import gitlab-shell
as a dependency without using a commit as version.

Fix this by changing the module path of gitlab-shell to instead be
`gitlab.com/gitlab-org/gitlab-shell/v14` and adjust all imports
accordingly.

Changelog: fixed

Fix check_ip argument when gitlab-sshd used with PROXY protocol

See merge request gitlab-org/gitlab-shell!616

When gitlab-sshd were used with the PROXY protocol, the `check_ip`
argument passed to `/api/v4/internal/allowed` was the Go remote
address, which is a host and port combination
(e.g. 127.0.0.1:12345). As a result, This prevents IP restrictions
from working properly on Rails. We fix this by stripping out the port
if it is present.

When OpenSSH is used, this is not an issue because the IP address
is extracted from `SSH_CONNECTION`.

Changelog: fixed

Before this change, the GitLab internal API could use a boolean
response field to indicate whether gitlab-shell should make
sidechannel connections go Gitaly. We now ignore that response field
and always use sidechannel connections.

This field will act as a feature flag that controls whether
gitlab-shell uses the old SSHUploadPack RPC or the new
SSHUploadPackWithSidechannel.

This reduces coupling between tests in
internal/gitlabnet/accessverifier/client_test.go, and will make it
easier to add new test cases in the future.

Note that the test server had a special behavior for the username
"second", but this was never used. So we removed that behavior in this
commit.