Update user_popover target prop type to any HTML element

What does this MR do?

This MR updates user_popover.vue to accept any given HTMLElement as the target prop. This resolves the admin/users issue #329518 (closed) using the suggested method from this comment #329518 (comment 564536775).

Testing & Setup

To confirm the changes on admin/users Vue table:

1. Enable the feature flag echo "Feature.enable(:vue_admin_users)" | rails c
2. View the Vue table at [GDK_HOST]/admin/users
3. Checkout default branch and repeat 1 & 2

Screenshots (strongly suggested)

Before	After

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 no user facing change because | only error was behind a development feature flag.
• [-] Documentation (if required)
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• 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

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• 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 #329518 (closed)