Bump gitlab-shell to 14.56.1 in 19.1 (gitlab-sshd CPU spin fix)

What does this MR do and why?

Backport of the gitlab-sshd CPU spin fix to 19.1.

GitLab 19.1 ships gitlab-shell 14.54.0, which pins golang.org/x/crypto v0.52.0. That crypto release has a spinloop bug in ssh mux SendRequest: gitlab-sshd instances with ClientAliveInterval > 0 can pin CPU cores at 100% and leak goroutines when a keepalive fires as its connection is torn down.

This MR bumps GITLAB_SHELL_VERSION to 14.56.1, which moves to golang.org/x/crypto v0.54.0 and carries the upstream fix. This is the same gitlab-shell release already shipping in 19.2 and 19.3.

• Fix (gitlab-shell): gitlab-shell!1517 (merged)
• Ref (work item): gitlab-shell#876 (closed)

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