harbor: strip 'robot$' prefix in robot account creation

This MR addresses the issue of duplicate 'robot$' prefixes when creating robot accounts in Harbor by stripping the prefix from the jinja2 template. The variable lima_kilo_harbor_robot_account_username in roles/harbor/defaults/main.yaml was not changed. This is because it's also used in the Tekton secret creation, where the full username including the 'robot$' prefix is required.

This change might need revision when updating to a newer Harbor version.