Avoid duplicated symbols for FollyConvert.h functions (#51393)

Summary:
Pull Request resolved: #51393

In D74738548 we copied RCTFollyConvert to Follyconvert and we moved it to a separate file.

The problem is that when we build react native, we now might have duplicated symbols, especially with prebuilds where the symbols are "flattened" together.

To fix this, we can have the old file point to the new file in order not to break our suers and have a single symbol anyway.

## Changelog:
[Internal] -

Reviewed By: sammy-SC

Differential Revision: D74883143

fbshipit-source-id: 98361307286cdc19c57b8f5c9f066aebea992896

Author: cipolleschi

packages/react-native/React/CxxUtils/RCTFollyConvert.h

@@ -7,15 +7,13 @@
 
 #import <Foundation/Foundation.h>
 
-#include <folly/dynamic.h>
+#include <react/utils/FollyConvert.h>
 
-namespace facebook::react {
+// This file has been moved to `react/utils/FollyConvert.h`.
+// We are keeping the header here for backwards compatibility.
 
-[[deprecated(
-    "This function is deprecated, please use /ReactCommon/react/utils/platform/ios/react/utils/FollyConvert.h instead")]]
-folly::dynamic convertIdToFollyDynamic(id json);
-[[deprecated(
-    "This function is deprecated, please use /ReactCommon/react/utils/platform/ios/react/utils/FollyConvert.h instead")]]
-id convertFollyDynamicToId(const folly::dynamic& dyn);
-
-} // namespace facebook::react
+#warning \
+    "[DEPRECATION] `React/RCTFollyconvert.h` is deprecated and will be removed in the future. \
+    If this warning appears due to a library, please open an issue in that library, and ask for an update. \
+    Please, replace the `#include <React/RCTFollyConver.h>` statements \
+    with `#include <react/utils/FollyConvert.h>`."