Undef true and false macros

Replacing `true` with `1` changes `JsonString("ABC", true)` into `JsonString("ABC", 1)`, so all static strings had a length of 1.

This is a workaround for arduino/ArduinoCore-sam#50
Fixes #2181

extras/tests/Misc/issue2181.cpp

@@ -0,0 +1,15 @@
+// ArduinoJson - https://arduinojson.org
+// Copyright © 2014-2025, Benoit BLANCHON
+// MIT License
+
+#define true 0x1
+#define false 0x0
+
+#include <ArduinoJson.h>
+#include <catch.hpp>
+
+TEST_CASE("Issue #2181") {
+  JsonDocument doc;
+  doc["hello"] = "world";
+  REQUIRE(doc.as<std::string>() == "{\"hello\":\"world\"}");
+}