fix(material/form-field): update outline gap when prefix changes

The position of the outline gap depends on the size of the prefix, but we weren't set up to update it when that happens.

Fixes #23863.

Author: crisbeto

src/material/form-field/form-field.html

@@ -16,7 +16,11 @@
       </div>
     </ng-container>
 
-    <div class="mat-form-field-prefix" *ngIf="_prefixChildren.length">
+    <div
+      class="mat-form-field-prefix"
+      *ngIf="_prefixChildren.length"
+      (cdkObserveContent)="updateOutlineGap()"
+      [cdkObserveContentDisabled]="appearance != 'outline'">
       <ng-content select="[matPrefix]"></ng-content>
     </div>