Adjust file header top if sidebar feature flag is off

What does this MR do and why?

• Fixes bug where file header on Overview tab wasn't in correct top location

Screenshots or screen recordings

How to set up and validate locally

• Check overview tab on MRs with both feature flag turned on and off

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.

app/assets/stylesheets/pages/notes.scss

@@ -334,7 +334,11 @@ $system-note-svg-size: 16px;
     border-radius: 0;
 
     @media (min-width: map-get($grid-breakpoints, md)) {
-      --initial-top: calc(#{$header-height} + #{$mr-tabs-height + 28px});
+      --initial-top: calc(#{$header-height} + #{$mr-tabs-height});
+
+      &.is-sidebar-moved {
+        --initial-top: calc(#{$header-height} + #{$mr-tabs-height + 28px});
+      }
 
       .with-performance-bar & {
         --top: 123px;