Removed JsonArray::is<T>(i) and JsonArray::set(i,v)

Removed `JsonObject::is<T>(k)` and `JsonObject::set(k,v)`
Replaced `T JsonArray::get<T>(i)` with `JsonVariant JsonArray::get(i)`
Replaced `T JsonObject::get<T>(k)` with `JsonVariant JsonObject::get(k)`

test/JsonObject/containsKey.cpp

@@ -8,7 +8,7 @@
 TEST_CASE("JsonObject::containsKey()") {
   DynamicJsonDocument doc;
   JsonObject obj = doc.to<JsonObject>();
-  obj.set("hello", 42);
+  obj["hello"] = 42;
 
   SECTION("returns true only if key is present") {
     REQUIRE(false == obj.containsKey("world"));