gen_script_man_pages.sh: add opt_perforce_sdp_backup.sh to the known-scripts list, and fix stale Classic-depot references in usage text.
Found while exercising release process Step 13/14 for real: SDP_Guide.Unix.adoc
already includes doc/gen/opt_perforce_sdp_backup.sh.man.txt, but this
generator never knew about that script, so the file was never produced --
'make clean all' in doc/ failed with a missing-include error. Added it,
matching the pattern of sibling Server/Unix/p4/common/bin/*.sh scripts.
Agent: Claude Code, Model: Claude Sonnet 5 (claude-sonnet-5), operating as bot_Claude_Anthropic.