Updates selector in QA::Page::Component::Dropdown#search_item

What does this MR do and why?

This MR updates the css selector used in QA::Page::Component::Dropdown#search_item to reflect the update to GlListbox

Fixes #386619 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Enable the vue_group_select feature flag.
2. git fetch && git checkout qa/fix_dropdown_select
3. cd qa && ./bin/qa Test::Instance::All -- qa/specs/features/ee/browser_ui/3_create/repository/project_templates_spec.rb
4. Repeat with feature flag disabled.

MR acceptance checklist

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