Migrate system_note.vue component in Rapid Diffs discussions

What does this MR do and why?

This MR migrates a simplified version of the system_note.vue component from merge requests to Rapid Diffs discussions.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable rapid_diffs_on_commit_show feature flag
2. Open any commit with diff discussions
3. Add ?rapid_diffs=true to the URL, follow it
4. Apply the patch below
5. Observe correct rendering of a system note

diff --git a/app/assets/javascripts/rapid_diffs/app/discussions/discussion_notes.vue b/app/assets/javascripts/rapid_diffs/app/discussions/discussion_notes.vue
--- a/app/assets/javascripts/rapid_diffs/app/discussions/discussion_notes.vue	(revision 5506201da4b91dfbf0745255215ef7a9508b23b1)
+++ b/app/assets/javascripts/rapid_diffs/app/discussions/discussion_notes.vue	(date 1764328328903)
@@ -42,7 +42,7 @@
 
 <template>
   <ul class="gl-list-none gl-p-0">
-    <system-note v-if="firstNote.system" :note="firstNote" />
+    <system-note v-if="firstNote.system || true" :note="firstNote" />
     <noteable-note
       v-else
       :note="firstNote"