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.

Gitaly project now properly respects module release flow
and includes a module suffix in the package name. It requires
to re-write all non-suffixed imports with suffixed of a specific
version of tha module. With proper module versioning we don't
need to use a 'replace' directive to point to specific commit
and can use semantic versioning for the gitaly dependency.

Part of: https://gitlab.com/gitlab-org/gitaly/-/issues/3177

Without these changes, Gitaly calls would not be linked to a parent
context. This means that they would have an unassociated correlationID,
and Gitaly RPC calls would not be cancel()ed by parent context
cancellation.

Changelog: fixed

In this case we don't need to propagate cleanup
function. It simplifies the code.

Refactors introspection of execution environment to rely on
per-connection state (`gitlab-shell`) or per request (`gitlab-sshd`)

Relates to https://gitlab.com/gitlab-org/gitlab-shell/-/issues/496