Update components to extend glimmer/component and enable ember/no-classic-components rule

Right now, there are a bunch of classic components in ember-api-docs. It would be nice to work through those and upgrade them all to use native syntax and extend from glimmer/component. The components requiring modifications are:

- [ ] app/components/api-index-filter.js
- [x] app/components/api-index.js
- [x] app/components/class-field-description.js
- [x] app/components/import-example.js
- [x] app/components/loading-spinner.js
- [x] app/components/search-input.js
- [x] app/components/search-input/dropdown-header.js
- [ ] app/components/search-input/dropdown-result.js
- [ ] app/components/search-input/dropdown.js
- [x] app/components/table-of-contents.js

Once we've done this we should then:

- [ ] re-enable the ESLint rule: 'ember/no-classic-components'

to prevent us from accidentally adding any legacy style components going forward.