Make wording in security report summaries more consistent

What does this MR do?

I started cleaning up the secure report summary generation code in preparation for #6079 (closed). It is better translatable now, and I reduced a lot of code duplication, and I actually fixed #7383 (closed) along the way 😅

Before	After	Description
		If the base has security vulnerabilities, but the base does not, we will say no new vulnerabilities in the grouped header. Additionally we removed security from the individual reports, as it seemed redundant
		Rewording of the error message
		Rewording of the message when both new and fixed vulnerabilities exist

What are the relevant issue numbers?

• See #6079 (closed)
• Closes #7383 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?