Make vulnerability list columns have consistent widths

What does this MR do?

When viewing the security dashboard:

the vulnerability list's columns will change their widths based on the content. This will make the table columns have different widths across different projects.

This MR sets explicit widths for the columns so that they are consistent between projects.

Before	After

The changes also follows this design:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done