Mark SSHUploadPack RPC as deprecated

For #6410 (closed)

SSHUploadPackWithSidechannel has been widely adopted. That version uses Sidechannel protocol, which was an optimization to address significant performance problems in the past. At the point of writing, all known clients migrated to the *sidechannel version. There's also no traffic on GitLab SaaS.

This commit marks that RPC as deprecated to prepare for future removal.