Use GlCollapsibleListbox for the create VSA form

What does this MR do and why?

Convert from using the deprecated GlDropdown and update to the new replacement component. Functionality remains the same. This replaces the Start event and End event dropdowns.

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. Create new group
2. Visit VSA page: Analyze -> Value Stream Analytics
3. Click New value stream
4. Select Create from no template
5. Interact with Start event/End event 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 #418059 (closed)