Make it clearer that cluster-enums-check is returning kUnknownEnumValue. (project-chip#31951)

Instead of hardcoding the returns via the ZAP template, just use the symbolic name.

Author: bzbarsky-apple

src/app/zap-templates/templates/app/cluster-enums-check.zapt

@@ -19,7 +19,7 @@ static auto __attribute__((unused)) EnsureKnownEnumValue(detail::{{asType label}
     {{/zcl_enum_items}}
         return val;
     default:
-        return static_cast<EnumType>({{first_unused_enum_value mode="first_unused"}});
+        return EnumType::kUnknownEnumValue;
     }
 }
 {{/unless}}
@@ -39,7 +39,7 @@ static auto __attribute__((unused)) EnsureKnownEnumValue({{asUpperCamelCase ../n
     {{/zcl_enum_items}}
         return val;
     default:
-        return static_cast<EnumType>({{first_unused_enum_value mode="first_unused"}});
+        return EnumType::kUnknownEnumValue;
     }
 }
 {{/unless}}