E2E test: require dropdown module in saml page object

What does this MR do and why?

Resolves test failures introduced in !136192 (comment 1661492916)

• #432647 (closed)
• #432651 (closed)
• #432648 (closed)
• #432649 (closed)
• #432650 (closed)

To test locally:

gdk config set omniauth.group_saml.enabled true && gdk reconfigure && gdk restart rails-web
QA_LOG_LEVEL=DEBUG QA_GITLAB_URL=http://192.168.4.31:3000 bundle exec rspec qa/specs/features/ee/browser_ui/5_package/dependency_proxy_sso_spec.rb

MR acceptance checklist

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