Fix positioning filter dropdown

What does this MR do and why?

Fixes the filter dropdown being cut off in the Vulnerability Report on smaller screens.

Changelog: fixed

EE: true

Screenshots or screen recordings

before	after

And here's a gif that shows how it reacts on screen resize:

How to set up and validate locally

Requirements:

1. You'll need an EE License
2. You'll need to have runners enabled (See $2408961 for setting up a runner)

Populate vulnerabilities

1. Import https://gitlab.com/gitlab-examples/security/security-reports
2. Go to CI / CD > Pipelines
3. Click on Run pipeline
4. Use default values and hit Run pipeline

This will trigger a new pipeline and your vulnerabilities will be created.

Note: If the runner that was previously working is not working anymore, simply go to /admin/runners and remove that runner and register a new one.

Steps to test this change

1. Go to Security & Compliance > Vulnerability Report
2. Change the window size and check the positioning of the filter dropdowns.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #374589 (closed)