Enable `group_members_filtered_search` feature flag by default

What does this MR do?

Related to #289911 (closed) and #228675 (closed)

In "Draft" because this isn't ready to be merged yet, getting a head start on the review so it can be merged once FF has been enabled on gitlab.com for a few days.

Enables the group_members_filtered_search feature flag by default.

Screenshots (strongly suggested)

Before	After

Mobile

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
  • One screenshot to be updated in a separate MR
• 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