Now the requests are verified via JWT

With this change we don't rely on the secret to either contain
a newline or not contain it.

Update Gitaly to v15

See merge request gitlab-org/gitlab-shell!676

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

When API isn't responsible or the resource is not accessible
(returns 404 or 403), then we shouldn't consider it as an error
on gitlab-sshd side

Remove deprecated function NewHTTPClient

See merge request gitlab-org/gitlab-shell!603