Update file locking test for new confirm modal

What does this MR do and why?

• Updates the file page object to handle the current gitlab-ui-based confirmation modal.
• Fixes a case of exception handling used as flow control.

Closes #349898 (closed)

How to set up and validate locally

bundle exec bin/qa Test::Instance::All http://gitlab.url -- qa/specs/features/ee/browser_ui/3_create/repository/file_locking_spec.rb

MR acceptance checklist

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