Fix autocomplete not having y-scroll

After changing flex-order the children did not have a max-height which
ended up in the child growing to outside of the screen instead of
being properly constrained.

Fix element-hq/element-web#18997

Author: Dariusz Niemczyk

res/css/views/rooms/_Autocomplete.scss

@@ -7,7 +7,6 @@
     background: $background;
     border-bottom: none;
     border-radius: 8px 8px 0 0;
-    max-height: 35vh;
     overflow: clip;
     display: flex;
     flex-direction: column;
@@ -64,6 +63,7 @@
     margin: 12px;
     height: 100%;
     overflow-y: scroll;
+    max-height: 35vh;
 }
 
 .mx_Autocomplete_Completion_container_truncate {