Resolve "Refactor jQuery dropdown implementation to GitLab UI GlDropdown in namespace_select.js"

What does this MR do?

This replaces the "Namespace Select" dropdown visible in Admin -> Projects. This is used for the namespace filter dropdown in the projects index, and the dropdown used when transferring a project to a new namespace.

The previous implementation relies on our deprecated jQuery-based dropdown library and is being replaced here with a Vue-based implementation which utilized GlDropdown. (see epic &4284)

Screenshots (strongly suggested)

Admin Projects Index	Admin Project View

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #284704 (closed)