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

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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/labels_select.js

Instructions

These uses of initDeprecatedJQueryDropdown are often bespoke implementations with custom logic, so there is no one-size-fits-all guide to replacing it. Look to other resolved issues within this epic for guidance and examples.

Part of &4284