Deprecate containsKey() in favor of doc["key"].is<T>()

See #2121

extras/tests/JsonObjectConst/CMakeLists.txt

@@ -3,7 +3,6 @@
 # MIT License
 
 add_executable(JsonObjectConstTests
-	containsKey.cpp
 	equals.cpp
 	isNull.cpp
 	iterator.cpp

extras/tests/JsonObjectConst/containsKey.cpp

@@ -1,40 +0,0 @@
-// ArduinoJson - https://arduinojson.org
-// Copyright © 2014-2024, Benoit BLANCHON
-// MIT License
-
-#include <ArduinoJson.h>
-#include <catch.hpp>
-
-#include "Literals.hpp"
-
-TEST_CASE("JsonObjectConst::containsKey()") {
-  JsonDocument doc;
-  doc["hello"] = 42;
-  auto obj = doc.as<JsonObjectConst>();
-
-  SECTION("supports const char*") {
-    REQUIRE(false == obj.containsKey("world"));
-    REQUIRE(true == obj.containsKey("hello"));
-  }
-
-  SECTION("supports std::string") {
-    REQUIRE(false == obj.containsKey("world"_s));
-    REQUIRE(true == obj.containsKey("hello"_s));
-  }
-
-#ifdef HAS_VARIABLE_LENGTH_ARRAY
-  SECTION("supports VLA") {
-    size_t i = 16;
-    char vla[i];
-    strcpy(vla, "hello");
-
-    REQUIRE(true == obj.containsKey(vla));
-  }
-#endif
-
-  SECTION("supports JsonVariant") {
-    doc["key"] = "hello";
-    REQUIRE(true == obj.containsKey(obj["key"]));
-    REQUIRE(false == obj.containsKey(obj["hello"]));
-  }
-}