Enable admin users Vue table by default

What does this MR do?

This MR enables the vue_admin_users feature flag by default, causing the admin/users table to render in Vue instead of HAML. Functionally the table remains unchanged, and the only change the user would see is the new GitLab UI components.

The feature flag and HAML views will be cleaned up in a follow-up MR !61129 (merged).

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required) No screenshots of the table in our docs to update
• 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 #290737 (closed)