Migrate Dropdown to Disclosure for DAST On-Demand scanner library

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR resolves #390438 (closed)

Under Dropdown migration epic legacy GlDropdown component is migrated to Pajamas compliant GlDisclosure

For this user case GlDisclosure is more suitable according to decision graph

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

BEFORE	AFTER

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Go to any project
2. Security & Compliance -> On-demand scans -> Create one if not exist -> open Scan library tab -> doted menu on right side of every row
3. If you want to see more Verify configuration option, run:

   Feature.enable(:dast_pre_scan_verification)

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 #390438 (closed)