Update approval rule E2E test to use new dropdown

What does this MR do and why?

Resolves #387607 (closed)

The element for selecting a user or group for an approval rule has changed, so the qa/specs/features/ee/browser_ui/3_create/merge_request/approval_rules_spec.rb E2E test is currently failing.

There were some difficulties interacting with this particular dropdown. With Capybara input and lots of data (like our live environments have), the filter would not filter correctly when searching for a group (even when using send_keys). Using the application manually I didn't face this issue, even when pasting in the input to mimic Capybara's #set.

The only way I could work around this was by sending an additional key press to trigger another filter. !108533 (diffs)

Tested this MR against GDK locally and against staging-ref

How to set up and validate locally

QA_GITLAB_URL=https://staging-ref.gitlab.com bundle exec rspec qa/specs/features/ee/browser_ui/3_create/merge_request/approval_rules_spec.rb

MR acceptance checklist

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