fix(material/expansion): inconsistent spacing for anchor buttons (#24882)

Fixes that the expansion panel was applying the margin only to `mat-button` set on a `button` node.

(cherry picked from commit a47869c)

Author: crisbeto

src/material/expansion/expansion-panel.scss

@@ -86,7 +86,7 @@
   justify-content: flex-end;
   padding: 16px 8px 16px 24px;
 
-  button.mat-button-base, button.mat-mdc-button-base {
+  .mat-button-base, .mat-mdc-button-base {
     margin-left: 8px;
 
     [dir='rtl'] & {