Revert 284471-misc-vulnerability-filters-refactor

What does this MR do?

On the pipeline security tab, a vulnerability list is supposed to be shown. However, a recent MR broke it so that the vulnerability list indefinitely shows a skeleton loader, due to a console error:

This MR reverts that MR, but keeps a few code improvements from the original MR.

Technical Details

The original MR switched from having the dropdowns themselves keep track of which items are selected, to having the querystring track which items are selected. However, this only works on pages that have Vue Router added to them, which the pipeline security tab does not. We discussed if we should add the router to that page as well, but there are some unanswered questions that could take some time to answer, and we need to fix this issue ASAP because it affects production and is 100% reproducible.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.