Update dropdown header component to glimmer

[geneukum]

Let's update the DropdownHeader component to extend glimmer/component instead of ember/component. Glimmer components offer a bunch of various advantages over classic components and are the direction that ember is heading in general.

While we're here, let's prefer single quotes to double quotes throughout our template file.

This should resolve the app/components/search-input/dropdown-header.js section of https://github.com/ember-learn/ember-api-docs/issues/805.