Move import_group_button selector to correct element

What does this MR do and why?

Move import_group_button selector to correct element

Fixes #419559 (closed)

In !122583 (merged) the dropdown was reworked. The qa-selector however needs to be on the button to import here, rather than on the dropdown caret.

How to set up and validate locally

1. bundle exec rspec qa/specs/features/browser_ui/1_manage/migration/gitlab_migration_group_spec.rb:26

MR acceptance checklist

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