Add simple dropdown component and change severity filter to use it

What does this MR do and why?

This MR adds simple_dropdown.vue and updates severity_filter.vue to use it. There are a bunch of vulnerability report filters that repeat the same boilerplate code, and simple_dropdown.vue refactors them into a reusable component.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #383346 (closed)