Refactor `ee/app/assets/javascripts/approvals/components/approvers_select.vue` to remove `approverUsers`

What does this MR do?

Fixes #2502 (closed)

Replaces Api.approverUsers in approvers_select.vue to use a newly added function Api.projectUsers instead. The latter function has been added in GitLab CE, and a backport has been added in this MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31801

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content
• backend Add support for attribute skip_users in the api endpoint /api/v4/project/:id/users

Performance 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

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