README.cmake.md: Document minimum required CMake version for --preset

CMakePreset.json is supported since 3.19, but we have a version
3 preset file, so need at least 3.21.

Github: #489
Change-Id: I44c555f6ffa08f2aee739c7f687fa3b678c86231
Signed-off-by: Frank Lichtenheld <[email protected]>
Acked-by: Gert Doering <[email protected]>
Message-Id: <[email protected]>
URL: https://www.mail-archive.com/[email protected]/msg28160.html
Signed-off-by: Gert Doering <[email protected]>

Author: flichtenheld

README.cmake.md

@@ -17,6 +17,10 @@ also makes cross-building with MinGW on Linux much simpler. However,
 builds are also possible by providing the build dependencies manually,
 but that might require specifying more information to CMake.
 
+You need at least CMake version 3.21 or newer for the `CMakePreset.json`
+file to be supported. Manual builds might be possible with older CMake
+versions, see `cmake_minimum_required` in `CMakeLists.txt`.
+
 If you're looking to build the full Windows installer MSI, take a look
 at https://github.com/OpenVPN/openvpn-build.git .
 
@@ -27,7 +31,7 @@ The following tools are expected to be present on the system, you
 can install them with a package manager of your choice (e.g.
 chocolatey, winget) or manually:
 
-* CMake
+* CMake (>= 3.21)
 * Git
 * Python (3.x), plus the Python module `docutils`
 * Visual Studion 17 (2022), C/C++ Enviroment
@@ -101,6 +105,7 @@ To build the Windows executables on a Linux system:
     git clone https://github.com/OpenVPN/openvpn.git
     export VCPKG_ROOT=$PWD/vcpkg
     cd openvpn
+    # requires CMake 3.21 or newer
     cmake --preset mingw-x64
     cmake --build --preset mingw-x64
     # unit tests are built, but no testPreset is provided. You need to copy