E2E test: click add protected branch button

What does this MR do and why?

This MR clicks the Add protected branch button to open the card. The button is new. This also changes the data-testid of this button.

Protected branches settings: Fix QA tests:

• Failure in browser_ui/3_create/repository/add_n... (#421578 - closed)
• Failure in ee/browser_ui/3_create/repository/re... (#421580 - closed)
• Failure in ee/browser_ui/3_create/repository/as... (#421581 - closed)

Related to Protected branches: Use GlCard (!128748 - merged)

Screenshots or screen recordings

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How to set up and validate locally

QA_LOG_LEVEL=DEBUG QA_GITLAB_URL=http://192.168.4.31:3000  bundle exec rspec qa/specs/features/browser_ui/3_create/repository/add_new_branch_rule_spec.rb:28

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #417912 (closed)