This explicitly imports gitaly v16 instead of v15 and removes the
old one from `go.mod`.

All the implementations of DNS discovery were done in this epic:
https://gitlab.com/groups/gitlab-org/-/epics/8971. Gitaly allows clients
to configure DNS discovery via dial option. This MR adds the exposed
dial options to client connection creation in Gitlab-shell.

Issue: https://gitlab.com/gitlab-org/gitaly/-/issues/4722
Changelog: added

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

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.

When gitlab-sshd has been introduced we've started running our
own SSH server. In this case we're able to cache and reuse
Gitaly connections and Registry.

It helps to reduce memory usage.