fix(material-experimental/mdc-select): panel losing max-height if menu is opened after select (#21341)

If an MDC-based `mat-menu` is opened after a `mat-select`, the select will lose
its `max-height`, because its selector specificity is too low and it gets overwritten
by the menu which uses the same set of mixins.

These changes resolve the issue by increasing the specificity.

Author: crisbeto

src/material-experimental/mdc-select/select.scss

@@ -68,7 +68,9 @@ $scale: 0.75 !default;
   margin: 0 $mat-select-arrow-margin;
 }
 
-.mat-mdc-select-panel {
+// Note that the `.mdc-menu-surface` is here in order to bump up the specificity
+// and avoid interference with `mat-menu` which uses the same mixins from MDC.
+.mdc-menu-surface.mat-mdc-select-panel {
   width: 100%; // Ensures that the panel matches the overlay width.
   max-height: $mat-select-panel-max-height;
   position: static; // MDC uses `absolute` by default which will throw off our positioning.