Gitlab report format broken after NVD API changes

[weyhmueller]

Describe the bug
PR #5920 introduced dependency check reports in Gitlab's own format. However PR #5978 came with a lot of refactoring and didn't respect that in the changes from #5920.

Version of dependency-check used
The problem occurs using version 9.0.0+ of the dependency check core.

Steps to reproduce the behavior:

1. Create a dependency check report in GITLAB format
2. View the resulting json file
3. Scroll down to then metadata of a vulnerability
4. See error: severity placeholder from the velocity template is not replaced anymore

Expected behavior
A Gitlab report file that contains information about the severity of the detected vulnerabilites.

For a fix see #6166.