Improve the fork message in the UI

This merge request rewords the the fork pipeline alert in the merge request page. (Source file: app/assets/javascripts/vue_merge_request_widget/mr_widget_options.vue)

Current message

Fork merge requests do not create merge request pipelines which validate a post merge result

Proposed message

Small tweaks are still likely, but the current proposal when this merge request was created was:

If the last pipeline ran in the fork project, it may be inaccurate. Before merge, we advise running a pipeline in this project.

Related issues

• Closes #208300 (closed)
• Related to #323061 (closed) where we're revising related UI strings