Replace data-qa-selectors with data-testid in branch_rules.rb

What does this MR do and why?

Replaces deprecated data-qa-selectors with data-testid in qa/qa/page/project/settings/branch_rules.rb.

Affected E2E test: qa/qa/specs/features/browser_ui/3_create/repository/add_new_branch_rule_spec.rb

Part of gitlab-org/quality/quality-engineering/team-tasks#1825 (closed)

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