Fix search debounce for Swimlanes board assignees

What does this MR do?

Currently, on Swimlanes boards, when we are searching for the assignee in the sidebar dropdown to add, the request is sent on every typed character without debouncing. This MR changes the logic of sending search query and refactors debounce to be a static value

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Related to #300272 (closed)