fix announcement bar overlap after signup (#22664)

Co-authored-by: Mattermost Build <build@mattermost.com>

webapp/channels/src/sass/routes/_signup.scss

@@ -3,7 +3,7 @@
 body {
     &.announcement-bar--fixed {
         .signup-header {
-            top: 24px;
+            top: 42px;
         }
     }
 }