Update MR tests for restructured mr widget changes

What does this MR do and why?

This adjusts the MR page object to handle the changes in !68565 (merged) as well the previous UI.

Test results

Tests passing with FF enabled:

• rebase_merge_request_spec
• approval_rules_spec
• restrict_push_protected_branch_spec

Tests passing with FF disabled:

• rebase_merge_request_spec
• approval_rules_spec
• restrict_push_protected_branch_spec

Tasks

• Remove the change to the feature flag definition file before merging.

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.