Fix sticky header on vulnerability reports

What does this MR do and why?

Fix sticky th on pipeline and instance vuln report

This commit fixes the sticky positioning of the table hader on both the pipeline- and instance-level vulnerability reports.

Changelog: fixed EE: true

Screenshots or screen recordings

Pipeline

Before	After
Screen_Recording_2023-12-11_at_8.46.52_am	Screen_Recording_2023-12-11_at_8.45.28_am

Instance

Before	After
Screen_Recording_2023-12-11_at_8.48.02_am	Screen_Recording_2023-12-11_at_9.04.01_am

How to set up and validate locally

1. Navigate to your instance's vulnerability report (e.g.: http://gdk.test:3000/-/security/vulnerabilities)
2. Select one or more vulnerabilities via the checkbox
3. Scroll down and verify that the sticky header is positioned correctly
4. Enable FF: echo "Feature.enable(:pipeline_security_dashboard_graphql)" | rails c
5. Navigate to a pipeline's vulnerability report
6. Repeat steps 2 and 3

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 #431951 (closed)