Can directly add approvers to approval rule

Issue: #30695 (closed)

Docs MR: !18972 (merged)

What does this MR do?

Remove extra step of requiring to "Add approvers" first. Now, you can auto add approvers when selecting from search result

Testing Case

• Go to the Project General Settings page > Expand MR approvals
• When you select the approver dropdown, it will automatically add it into the approver list
• You can search for approver, and when you select it, it will automatically add it into the approver list

Appearance

Empty	Dropdown	Search	Click on a result and it's added to the list

Screenshots

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

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

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

Closes #30695 (closed)