fix(material/datepicker): use rounded shape for calendar focus indicators (angular#33524)

Makes the strong focus indicators in the calendar rounded to match the calendar cells.

Relates to angular#33517.

Author: crisbeto

src/material/datepicker/calendar-body.scss

@@ -261,6 +261,10 @@ $fallbacks: m3-datepicker.get-tokens();
     position: absolute;
   }
 
+  &::before {
+    border-radius: 50%;
+  }
+
   @include cdk.high-contrast {
     border: none;
   }