Setting TCP KeepAlives for Tailscale SSH connections results in them
unnecessarily disconnecting. However, we can't turn them off completely
as that would mean we start leaking sessions waiting for a peer to come
back which may have gone away forever (e.g. if the node was deleted from
the tailnet during a session).
Updates #5021
Signed-off-by: Maisem Ali <maisem@tailscale.com>