fix(material/core): reintroduce opacity in elevation mixin (#26015)

During the switch to MDC the opacity parameter to the `elevation` mixin was removed which is a breaking change. It also isn't possible to achieve the same result by providing an `rgba` color, because MDC changes the color's opacity.

Fixes #26004.

Author: crisbeto

src/material/core/style/_elevation.scss

@@ -36,20 +36,22 @@ $prefix: 'mat-elevation-z';
 
 // Applies the correct css rules to an element to give it the elevation specified by $zValue.
 // The $zValue must be between 0 and 24.
-@mixin elevation($zValue, $color: $color) {
-  @if meta.type-of($color) == color {
+@mixin elevation($zValue, $color: $color, $opacity: null) {
+  @if meta.type-of($color) == color and $opacity == null {
     @include mdc-elevation.elevation($zValue, $color);
   }
   @else {
     // Copied from @material/elevation/_elevation-theme.scss#_box-shadow
     // TODO(mmalerba): Add support for graceful handling of CSS var color to MDC.
+    $shadow-color:
+      if(meta.type-of($color) == color and $opacity != null, rgba($color, $opacity), $color);
     $umbra-z-value: map.get(mdc-elevation.$umbra-map, $zValue);
     $penumbra-z-value: map.get(mdc-elevation.$penumbra-map, $zValue);
     $ambient-z-value: map.get(mdc-elevation.$ambient-map, $zValue);
     $box-shadow: (
-      #{'#{$umbra-z-value} #{$color}'},
-      #{'#{$penumbra-z-value} #{$color}'},
-      #{$ambient-z-value} $color
+      #{'#{$umbra-z-value} #{$shadow-color}'},
+      #{'#{$penumbra-z-value} #{$shadow-color}'},
+      #{$ambient-z-value} $shadow-color
     );
     @include mdc-elevation.shadow($box-shadow);
   }