chore: Remove Elevation from Brand and Primary Buttons

The Brand and Primary Buttons currently display a shadow/elevation
effect, which is inconsistent with the Paragon Elm Theme guidelines.
This deviation from the design standard needs to be addressed by
removing the shadow/elevation from these buttons.

Fixes: LEARNER-10366

core/src/main/java/org/openedx/core/ui/ComposeCommon.kt

@@ -1087,6 +1087,7 @@ fun OpenEdXBrandButton(
             backgroundColor = backgroundColor,
             disabledBackgroundColor = backgroundColor.copy(alpha = 0.3f)
         ),
+        elevation = null,
         enabled = enabled,
         onClick = onClick,
     ) {