Add export_path in order to make export api

What does this MR do and why?

Add export_path to this endpoint in order to make export api to consider relative_url.

Related issue: #384884 (closed)

How to set up and validate locally

1. Host Gitlab application on a relative url. https://docs.gitlab.com/ee/install/relative_url.html
2. Go to a project's vulnerability report with a few vulnerabilities.
3. Click on export to csv button.

Alternatively:

1. Setup GDK by following these steps. It might require gdk reconfigure.
2. Go to a project's vulnerability report with a few vulnerabilities.
3. Click on export to csv button.

Another option of validation is through rails console:

Currently the vulnerabilitiesExportEndpoint used by CsvExportButton does not include the relative aspect of the host url.

pry(main)> app.api_v4_security_projects_vulnerability_exports_path(id: 46)
=> "/api/v4/security/projects/46/vulnerability_exports"

Therefore calling expose_path when setting vulnerabilities_export_endpoint ensures that the relative aspect of the host url is considered.

pry(main)> expose_path(app.api_v4_security_projects_vulnerability_exports_path(id: 46))
=> "/gitlab/api/v4/security/projects/46/vulnerability_exports"

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.