Enable `vue_project_members_list` feature flag by default

📖 What does this MR do?

Related &4901 (closed) and #299954 (closed)

Enables the vue_project_members_list feature flag by default. This feature flag has been enabled for 50% of projects for the last couple of days. Hoping to enable for all projects ASAP, but waiting for there to be no production incidents. Looking to get a head start on review of this so that it can be merged in the next few days and included in the %13.9 release.

💻 Local testing

1. Create a project
2. Navigate to Project -> Members

📷 Screenshots

No visual changes, GIF for context

🚦 Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
  • https://docs.gitlab.com/ee/user/project/members/
• 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