Fix sticky header z-index

What does this MR do and why?

Needs to fix the z-index of the sticky header in vulnerability report page

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After
	Screen_Recording_2024-08-01_at_17.28.39

How to set up and validate locally

1. Upload a GitLab Ultimate license
2. Enable the feature flag (echo "Feature.enable(:vulnerability_code_flow)" | rails c)
3. Clone the code flow project and run his pipeline to display vulnerabilities
4. Navigate to the project => Secure => Vulnerability report => Select the first vulnerability to navigate to the vulnerability details page
5. Apply the following patch to show the new features

Patch

diff --git a/ee/app/assets/javascripts/vulnerabilities/components/vulnerability.vue b/ee/app/assets/javascripts/vulnerabilities/components/vulnerability.vue
index ca4110274476..319e8f7180ae 100644
--- a/ee/app/assets/javascripts/vulnerabilities/components/vulnerability.vue
+++ b/ee/app/assets/javascripts/vulnerabilities/components/vulnerability.vue
@@ -56,6 +56,7 @@ export default {
       },
     },
     showCodeFlowTabs() {
+      return true
       return (
         this.glFeatures.vulnerabilityCodeFlow && !isEmpty(this.vulnerability.details.codeFlows)
       );
diff --git a/ee/app/assets/javascripts/vulnerabilities/components/vulnerability_details.vue b/ee/app/assets/javascripts/vulnerabilities/components/vulnerability_details.vue
index bfbc47e4c041..72fd47398163 100644
--- a/ee/app/assets/javascripts/vulnerabilities/components/vulnerability_details.vue
+++ b/ee/app/assets/javascripts/vulnerabilities/components/vulnerability_details.vue
@@ -221,6 +221,7 @@ export default {
       return this.vulnerability.location?.file && this.vulnerability.location?.startLine;
     },
     showCodeFlowButton() {
+      return true
       return (
         this.glFeatures.vulnerabilityCodeFlow && !isEmpty(this.vulnerability.details.codeFlows)
       );

6. Navigate to code flow tab
7. Scroll down and verify that you have a sticky header looking as needs to.

Related to #476550 (closed)