cmake: Fix SLINT_EMBED_RESOURCES

* Do not use the initialize_from feature introduced in cmake 3.23

api/cpp/docs/cmake.md

@@ -42,7 +42,7 @@ First you need to install the prerequisites:
 * Install Rust by following the [Rust Getting Started Guide](https://www.rust-lang.org/learn/get-started). If you already
   have Rust installed, make sure that it's at least version 1.60 or newer. You can check which version you have installed
   by running `rustc --version`. Once this is done, you should have the ```rustc``` compiler and the ```cargo``` build system installed in your path.
-* **[cmake](https://cmake.org/download/)** (3.23 or newer)
+* **[cmake](https://cmake.org/download/)** (3.21 or newer)
 * A C++ compiler that supports C++20 (e.g., **MSVC 2019 16.6** on Windows)
 
 You can include Slint into your CMake project using CMake's

api/cpp/docs/getting_started.md

@@ -15,7 +15,7 @@ target in two steps:
 A minimal CMake `CMakeLists.txt` file looks like this:
 
 ```cmake
-cmake_minimum_required(VERSION 3.23)
+cmake_minimum_required(VERSION 3.21)
 project(my_application LANGUAGES CXX)
 
 # Note: Use find_package(Slint) instead of the following three commands,