Reduce height of instance system message

- Reduces height of instance system header and footer to 16px
- Pulls the system message up to better fit the container

app/assets/stylesheets/framework/system_messages.scss

@@ -12,7 +12,7 @@
 
   p {
     @include str-truncated(100%);
-    margin-top: 0;
+    margin-top: -1px;
     margin-bottom: 0;
   }
 }

app/assets/stylesheets/framework/variables.scss

@@ -277,7 +277,7 @@ $general-hover-transition-duration: 100ms;
 $general-hover-transition-curve: linear;
 $highlight-changes-color: rgb(235, 255, 232);
 $performance-bar-height: 35px;
-$system-header-height: 35px;
+$system-header-height: 16px;
 $system-footer-height: $system-header-height;
 $flash-height: 52px;
 $context-header-height: 60px;

changelogs/unreleased/58410-change-pixel-size-of-instance-header-footer-message-to-16px.yml

+---
+title: Reduce height of instance system header and footer
+merge_request: 25752
+author:
+type: changed