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

See #2121

extras/tests/JsonVariantConst/CMakeLists.txt

@@ -4,7 +4,6 @@
 
 add_executable(JsonVariantConstTests
 	as.cpp
-	containsKey.cpp
 	is.cpp
 	isnull.cpp
 	nesting.cpp

extras/tests/JsonVariantConst/containsKey.cpp

@@ -1,41 +0,0 @@
-// ArduinoJson - https://arduinojson.org
-// Copyright © 2014-2024, Benoit BLANCHON
-// MIT License
-
-#include <ArduinoJson.h>
-#include <stdint.h>
-#include <catch.hpp>
-
-#include "Literals.hpp"
-
-TEST_CASE("JsonVariantConst::containsKey()") {
-  JsonDocument doc;
-  doc["hello"] = "world";
-  JsonVariantConst var = doc.as<JsonVariant>();
-
-  SECTION("support const char*") {
-    REQUIRE(var.containsKey("hello") == true);
-    REQUIRE(var.containsKey("world") == false);
-  }
-
-  SECTION("support std::string") {
-    REQUIRE(var.containsKey("hello"_s) == true);
-    REQUIRE(var.containsKey("world"_s) == false);
-  }
-
-#ifdef HAS_VARIABLE_LENGTH_ARRAY
-  SECTION("supports VLA") {
-    size_t i = 16;
-    char vla[i];
-    strcpy(vla, "hello");
-
-    REQUIRE(true == var.containsKey(vla));
-  }
-#endif
-
-  SECTION("support JsonVariant") {
-    doc["key"] = "hello";
-    REQUIRE(var.containsKey(var["key"]) == true);
-    REQUIRE(var.containsKey(var["foo"]) == false);
-  }
-}