Prepare HPSS DSI 2.9 "restart" beta

[JasonAlt]

• [x] Verify fix for final restart marker (globusonline/globus-connect-gridftp-server#40)
• [x] Update packaging and docs to require min globus-gridftp-server-12.23
• [x] Perform non-test transfer tests
• [x] Tag pre release vs 2.9rc4
• [x] Build, upload, sign new packaging
• [x] Rebase public connector documents
• [x] Send update to customer site
• [x] Add instructions to enable unstable GT6 repo