Add QA selector to the Vue label dropdown on merge request form

What does this MR do and why?

#415717 (closed)

This change makes the E2E tests compatible when visible_label_selection_on_metadata is enabled and disabled.

• an extra selector has been added
• click_on is used as it handles buttons and links (when visible_label_selection_on_metadata is enabled the label element is a button)
• use a less generic label name (previously was label)
• convert some data-qa-selectors to data-testid

How to set up and validate locally

Feature.disable :visible_label_selection_on_metadata
bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/create_merge_request_spec.rb
Feature.enable :visible_label_selection_on_metadata
bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/create_merge_request_spec.rb

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.