Switch to using GlSort for group members view

What does this MR do?

Related to #228675 (closed) and #292042 (closed)

As a follow-up to !49168 (comment 461485637) this MR switches from using GlDropdown to GlSort for the sorting on the group members view

Local testing

1. Enable the group_members_filtered_search feature flag
   • bin/rails console
   • Feature.enable(:group_members_filtered_search)
2. Create a group
3. Add members in Group -> "Members"
4. Use the filter bar and sort dropdown

Screenshots (strongly suggested)

Desktop

Before	After

Mobile

Before	After

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - Not needed, behind a feature flag
• [-] 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