Add severity override support to MR security widget

What does this MR do and why?

When comparing security findings in merge request reports, prioritize existing vulnerability severities over the original report severities. This ensures that manually overridden vulnerability severities are correctly reflected in the merge request security widget:

Related issue

Add Severity Override Metadata to MR Widget Calls (#511232 - closed) • Miki Amos, Gal Katz • 17.10