Why some files in data directory of a Postgresql cluster are symlink to files of another cluster?
This is the result of a pg_upgrade with the -k option, and these are not symbolic links, but hard links.
If you didn't manage to start the old cluster after pg_upgrade everything is fine. You just forgot to remove the old cluster after upgrading. Do that right now:
rm -rf /var/lib/postgresql/9.6/main

If you use -k, the upgrade process doesn't copy, but link the files. That makes the upgrade much faster, but renders the old cluster unusable.