Refactor jQuery dropdown implementation to GitLab UI GlDropdown in search_autocomplete.js

The following component utilizes our old deprecated jQuery-based dropdown implementation and should be replaced with a new implementation based upon GitLab UI's GlDropdown component:

• app/assets/javascripts/search_autocomplete.js

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This component is being refactored with #297396 (closed) and will be purged once that is complete. We can close this issue once the feature flag is removed.

Part of &4284