Support new delayed deletion setting

Support disabling soft-delete for v15.3 and up. With v15.3 option to set deletion_adjourned_period=0 was removed, now we should set delayed_project_deletion and delayed_group_deletion to false. Additionally adding a fix to ensure disable soft deletion runs only once when needed.

Also bumps Generator version to 1.0.26.

Code was validated against 15.3 and 14.10 GitLab instances.

Closes #525 (closed)