[License Compliance MR Widget Extension] - Frontend - Specify `for the source branch only` header messaging

Description

Conditionally add text for source branch only if existing licenses are detected in the base branch

Current iteration of License MR widget extension

What we need to add to the current iteration from the legacy component

Implementation Plan

• In ee/app/assets/javascripts/vue_merge_request_widget/extensions/license_compliance/index.js update the summary logic to show the for source branch only text.

• Update unit tests

Testing

Frontend engineer will update the unit tests to check for this text. In accordance with shift left, no E2E/system test update https://docs.gitlab.com/ee/development/testing_guide/testing_levels.html#consider-not-writing-a-system-test